<PAGE>

                                                                    EXHIBIT 4.11

                                AMENDMENT TO THE
                       LIMITED LIABILITY COMPANY AGREEMENT
               OF RELIANT ENERGY MID-ATLANTIC POWER HOLDINGS, LLC

                  This Amendment (this "Amendment") to the Limited Liability
Company Agreement (the "Agreement") of Reliant Energy Mid-Atlantic Power
Holdings, LLC, a Delaware limited liability company (the "Company"), is made and
entered into effective as of August 1, 2000.

                  WHEREAS, Reliant Energy Northeast Generation, Inc.,
("Generation") as the sole holder of all the member interests in the Company,
desires to amend the Agreement as set forth herein.

                  NOW THEREFORE, pursuant to Section 15 of the Agreement,
Generation, as sole member of the Company, hereby amends the following sections
of the Agreement as set forth below:

                  1. Section 6 of the Agreement is hereby amended in its
entirety to read as follows:

        "6.       Management.

         Member.           Except as otherwise provided under this Agreement,
         the Member shall not perform any act on behalf of the Company, incur
         any expense, obligation or indebtedness of any nature on behalf of the
         Company, or in any manner participate in the management of the Company
         or receive or be credited with any amounts.

         Managers.         The powers of the Company shall be exercised by or
         under the authority of, and the business and affairs of the Company
         shall be managed under, its Managers. In addition to the powers and
         authorities expressly conferred by this Agreement upon the Managers,
         the Managers may exercise all such powers of the Company and do all
         such lawful acts and things as are not directed or required to be
         exercised or done by the Members by the Delaware Act, the Certificate
         of Formation of the Company or this Agreement.

         Number and Qualifications of Managers. The Member shall have the right
         to take action pursuant to a meeting of the Member or unanimous written
         consent of the Member to designate one or more Managers and to remove,
         replace or fill any vacancy occurring for any reason of any Manager.
         The number of Managers of the Company shall be two; but the number of
         Managers may be changed by unanimous agreement of the Member. Managers
         need not be residents of the State of Delaware or Members of the
         Company. The Managers, in their discretion, may elect a chairman of the
         Managers who shall preside at any meetings of the Managers.

         Powers of the Managers. Without limiting the generality of this Section
         6, the Managers

<PAGE>

         shall have power and authority, acting in concert in accordance with
         this Agreement, to cause the Company to do and perform all acts as may
         be necessary or appropriate to the conduct of the Company's business.

         Delegation of Rights and Powers to Manage. The Managers shall have the
         power and authority to delegate to one or more other persons the
         Managers' rights and powers to manage and control the business and
         affairs of the Company, including to delegate to agents, officers and
         employees of a member or manager of the Company and to delegate by a
         management agreement or another agreement with, or otherwise to, other
         persons. Without limiting the generality of the preceding sentence, the
         Managers may delegate their rights and powers to manage pursuant to
         resolutions adopted by the Managers.

         Loans to Company. Nothing in this Agreement shall prevent any Member
         from making secured or unsecured loans to the Company by agreement with
         the Company.

         Borrowing. In the event that the Company, in order to discharge costs,
         expenses or indebtedness, requires funds in excess of the funds
         provided by Capital Contributions of the Members and by revenues, the
         Managers shall be authorized, at any time and from time to time, to
         cause the Company to borrow additional funds, as shall in the judgment
         of the Managers be sufficient for such purposes and upon such terms as
         the Managers may deem advisable."

                  2.       Section 7 of the Agreement is hereby amended in its
entirety to read as follows:

         "7.      Officers. The Managers of the Company may elect the officers
         of the Company, who shall hold offices specified by the Managers for
         such terms and shall exercise such powers and perform such duties as
         shall be determined from time to time by the Managers; and each officer
         of the Company shall hold office until his successor is chosen and
         qualified or until his earlier resignation or removal. Any officer
         elected by the Managers may be removed at any time by the affirmative
         vote of the Managers. Any vacancy occurring in any office of the
         Company may be filled by the unanimous agreement of the Managers. The
         salaries of all officers of the Company shall be fixed by the Managers
         but must be approved by the Members."

                  3.       Section 8 of the Agreement is hereby amended in its
entirety to read as follows:

         "8.      Capital Contributions. The Member may make capital
         contributions to the Company from time to time, but shall not be
         required to make any capital contributions. A class of equity interests
         denominated the "Common Shares" is hereby designated as the sole class
         of equity interests of the Company. Each issued and outstanding Common
         Share shall at any time represent that undivided portion of all of the
         rights, duties, obligations and ownership interests in the Company in
         proportion to the total number of Common Shares outstanding at such
         time. The Company shall issue to the Member 1,000 Common Shares to
         reflect the capital contribution heretofore made by the Member. Upon

<PAGE>

         issuance, such Common Shares shall be validly issued and outstanding,
         fully paid and nonassessable."

                                        3

<PAGE>

         IN WITNESS WHEREOF, the undersigned as sole member of the Company has
caused this Amendment to be executed on the date and year first above written.

                                      RELIANT ENERGY NORTHEAST GENERATION, INC.

                                      By: /s/ J. Douglas Divine
                                          -------------------------------------
                                          J. Douglas Divine
                                          President


                                        4

<PAGE>

                                AMENDMENT TO THE
                       LIMITED LIABILITY COMPANY AGREEMENT
                       OF SITHE PENNSYLVANIA HOLDINGS LLC

                  This Amendment (this "Amendment") to the Limited Liability
Company Agreement (the "Agreement") of Sithe Pennsylvania Holdings LLC, a
Delaware limited liability company (the "Company"), is made and entered into
effective as of May 12, 2000.

                  WHEREAS, pursuant to the Purchase Agreement dated as of
February 19, 2000, among Reliant Energy Power Generation, Inc., Reliant Energy,
Incorporated, Sithe Energies, Inc., and Sithe Northeast Generating Company, Inc.
(as amended and supplemented, the "Purchase Agreement"), Reliant Energy
Northeast Generation, Inc., a Delaware corporation ("Generation") has acquired
(the "Acquisition") all of the limited liability company interests of the
Company; and

                  WHEREAS, in connection with the Acquisition, Generation, as
the sole holder of all the member interests in the Company, desires to amend the
Agreement as set forth herein.

                  NOW THEREFORE, pursuant to Section 15 of the Agreement,
Generation, as sole member of the Company, hereby amends the following sections
of the Agreement as set forth below:

                  1. Section 2 of the Agreement is hereby amended in its
entirety to read as follows:

         "2. Name: The name of the Company is "Reliant Energy Mid-Atlantic Power
         Holdings, LLC."

                  2. Section 3 of the Agreement is hereby amended in its
entirety to read as follows:

         "3. Member. Reliant Energy Northeast Generation, Inc. (the "Member") is
         the sole member of the Company."

                  3. The last sentence of Section 4 of the Agreement is hereby
amended in its entirety to read as follows:

         "The mailing address and principal business office of the Company shall
         be do Reliant Energy Northeast Generation, Inc., 1111 Louisiana,
         Houston, Texas 77002."

<PAGE>

                  IN WITNESS WHEREOF, the undersigned as sole member of the
Company has caused this Amendment to be executed on the date and year first
above written.

                                       RELIANT ENERGY NORTHEAST GENERATION, INC.

                                       By: /s/ J. Douglas Divine
                                           ----------------------
                                           J. Douglas Divine
                                           President

<PAGE>

                                  AMENDMENT TO
                       LIMITED LIABILITY COMPANY AGREEMENT
                                       OF
                         SITHE PENNSYLVANIA HOLDINGS LLC

                  This Amendment to Limited Liability Company Agreement (this
"Amendment"), of Sithe Pennsylvania Holdings LLC, a Delaware limited liability
company (the "Company") is effective as of November 23, 1999.

                  WHEREAS, Sithe Northeast Generating Company, Inc. ("Northeast
Generating") has been admitted as a member of the Company;

                  WHEREAS, Sithe Mid-Atlantic, Inc. is withdrawing as a member
of the Company;

                  WHEREAS, Northeast Generating shall thereafter be the sole
member of the Company; and

                  WHEREAS, pursuant to Section 15 of the Limited Liability
Company Agreement of Sithe Pennsylvania Holdings LLC (the "LLC Agreement"), the
LLC Agreement may only be amended or modified by a written instrument executed
by the member;

                  NOW, THEREFORE:

                  1.       Section 3 of the LLC Agreement is hereby amended by
deleting Section 3 in its entirety and by replacing it with the following:

                           3.       Member. Sithe Northeast Generating Company,
                                    Inc. (the "Member") is the sole
                                    member of the Company.

                  2.       Except as amended hereby, the LLC Agreement shall
remain in full force and effect.

                  IN WITNESS WHEREOF, the undersigned has executed this
Amendment as of the date first written above.

                                   Sithe Northeast Generating Company, Inc.,
                                   as sole member

                                   By: /s/ Hyun Park
                                       -------------
                                   Name: Hyun Park
                                   Title: Senior VP, General Counsel & Secretary

<PAGE>

                       LIMITED LIABILITY COMPANY AGREEMENT
                                       OF
                         SITHE PENNSYLVANIA HOLDINGS LLC

                  This LIMITED LIABILITY COMPANY AGREEMENT (the "Agreement") of
Sithe Pennsylvania Holdings LLC, a Delaware limited liability company (the
"Company"), is effective as of December 28, 1998.

                  1.       Formation. The Company has been formed as a single
member limited liability company pursuant to and in accordance with the Delaware
Limited Liability Company Act, 6 Del. C Section 18-101, et seq., as it may be
amended from time to time (the "Act"), by the filing of the Certificate of
Formation of the Company (the "Certiflcate") with the Secretary of State of the
State of Delaware on the date first written above. The rights and obligations of
the Member (as defined below) and the administration and termination of the
Company shall be governed by this Agreement and the Act. In the event of any
inconsistency between any terms and conditions contained in this Agreement and
any non-mandatory provisions of the Act, the terms and conditions contained in
this Agreement shall govern.

                  2.       Name. The name of the Company is "Sithe Pennsylvania
Holdings LLC".

                  3.       Member. Sithe Mid-Atlantic, Inc. (the "Member") is
the sole member of the Company.

                  4.       Registered Office and Agent. The registered office
and registered agent of the Company in the State of Delaware shall be as the
Company designates on its Certificate of Formation filed with the Secretary of
State of the State of Delaware, as such Certificate may be amended from time to
time. The Company may have such other offices as the Member may designate from
time to time. The mailing address and principal business office of the Company
shall be do Sithe Energies, Inc., 450 Lexington Avenue, New York, New York
10017.

                  5.       Purpose. The purpose of the Company is to engage in
any and all lawful businesses or activities in which a limited liability company
may be engaged under applicable law.

                  6.       Management. The Member shall be authorized to make
all decisions and to take all actions it determines necessary, advisable or
desirable relating to the business, affairs, investments, and properties of the
Company including, without limitation, the formation of or investment in, such
subsidiary or affiliate companies of the Company as it determines advisable or
desirable.

                  7.       Officers. The Member may delegate its authority to
act on behalf of the Company and to manage the business affairs of the Company
to one or more officers of the Company appointed by the Member. The Member may
from time to time create offices of the Company, designate the powers that may
be exercised by such office, and appoint, authorize and

<PAGE>

empower any person as an officer of the Company ("Officer") to direct such
office. The initial offices of the Company and the initial Officers of the
Company serving in such offices are set forth on Annex 1 which is attached
hereto and made a part hereof. The Member may remove any Officer at any time and
may create, empower and appoint such other Officers of the Company as the Member
may deem necessary or advisable to manage the day-to-day business affairs of the
Company. To the extent delegated by the Member, the Officers shall have the
authority to act on behalf of, bind and execute and deliver documents in the
name of and on behalf of the Company. No such delegation shall cause the Member
to cease to be a member. Except as otherwise expressly provided in this
Agreement or required by any non-waivable provision of the Act or other
applicable law, no person other than the Member and such Officers designated by
the Member shall have any right, power, or authority to transact any business in
the name of the Company or to act for or on behalf of or to bind the Company.

                  8.       Capital Contributions. The Member may make capital
contributions to the Company from time to time, but shall not be required to
make any capital contributions.

                  9.       Allocations: Distributions. Each item of income,
gain, loss, deduction and credit of the Company shall be allocated 100% to the
Member. Each distribution of cash or other property by the Company shall be made
100% to the Member. Distributions shall be made to the Member at the times and
in the amounts determined by the Member.

                  10.      Limited Liability of the Member. The Member shall
have no liability for obligations or liabilities of the Company unless such
obligations or liabilities are expressly assumed by the Member in writing.

                  11.      Indemnification. The Member and each person who is or
has agreed to become a director or officer of the Company, or each such person
who serves or has agreed to serve at the request of the Company as a director or
officer of another corporation, limited liability company, partnership, joint
venture, trust or other enterprise (including the heirs, executors,
administrators or estate of such person), shall be indemnified by the Company to
the full extent permitted by the Limited Liability Company Act of the State of
Delaware or any other applicable laws as now or hereafter in effect. The right
to indemnification conferred in this Section 11 shall include the right to be
paid by the Company the expenses incurred in defending or otherwise
participating in any proceeding in advance of its final disposition. Without
limiting the generality or effect of the foregoing, the Company may enter into
one or more agreements with any person which provide for indemnification greater
or different than that provided in this Section. No amendment to this Section 11
shall apply to or have any effect on the right to indemnity permitted or
authorized hereunder for or with respect to claims asserted before or after such
amendment arising from acts or omissions occurring in whole or in part before
the effective date of such amendment.

                  12.      Dissolution. The Company shall dissolve and its
business and affairs shall be wound up upon the written consent of the Member or
the entry of a decree of judicial dissolution under Section 18-802 of the Act.
Upon the dissolution of the Company, the affairs of the Company shall be
liquidated forthwith. The assets of the Company shall be used first to pay

                                        2

<PAGE>

or provide for the payment of all of the debts of the Company, with the balance
being distributed to the Member.

                  13.      Assignment. The Member may assign in whole or in part
its limited liability company interest in the Company.

                  14.      Admission of Additional Members. The Member may admit
additional members in its discretion.

                  15.      Amendment. This Agreement may be amended or modified
from time to time only by a written instrument executed by the Member.

                  16.      No Third-Party Beneficiaries. None of the provisions
of this Agreement shall be for the benefit of or enforceable by any creditor of
the Company or by any creditor of the Member.

                  17.      GOVERNING LAW. THIS AGREEMENT SHALL BE CONSTRUED IN
ACCORDANCE WITH, AND GOVERNED BY, THE LAWS OF THE STATE OF DELAWARE WITHOUT
GIVING EFFECT TO ANY CHOICE OR CONFLICT OF LAW PROVISION OR RULE (WHETHER OF THE
STATE OF DELAWARE OR ANY OTHER JURISDICTION) THAT WOULD CAUSE THE APPLICATION OF
THE LAWS OF ANY JURISDICTION OTHER THAN THE STATE OF DELAWARE.

         [THE REMAINDER OF THIS PAGE HAS BEEN LEFT BLANK INTENTIONALLY.]

                                        3

<PAGE>

                                          Sithe Mid-Atlantic, Inc.,
                                          as sole member

                                          By: /s/ David L. Tohir
                                             -----------------------------------
                                          Name: David L. Tohir
                                               ---------------------------------
                                          Title: President
                                                --------------------------------


                                        4

<PAGE>

                                     ANNEX 1

                                    OFFICERS

<TABLE>
<CAPTION>
         NAME                            TITLE
         ----                            -----
<S>                                    <C>
Richard J. Cronin, III                 President
Richard J. Cronin, III                 Secretary
Sandra Manilla                         Treasurer
</TABLE>