Second, she must sign papers to eliminate your brother's legal ability to grab control via an involuntary Guardianship. Once done, your sister will have appointed an assistant to make decisions as her condition gradually worsens. The second goal is accomplished by signing a Declaration of Guardian. This document contains her instructions on who, if anyone, should become her Guardian if the need arises. It is this ability to disqualify individuals that makes the Declaration of Guardian especially powerful.
Guardianship is never a voluntary legal procedure.
Generally, it is used as a last resort when no voluntary pre-planning, like a Durable Power of Attorney, had been done. You and your sister cannot control your brother, but a Declaration of Guardian can state that he cannot ever become your sister's Guardian. He would gain nothing by starting Guardianship proceedings. If he does make an attempt, the court would appoint the person named by your sister, and he would not gain any control. If he does not make an attempt, then your sister's Durable Powers of Attorney will continue to be valid and her voluntarily selected representative can take care of her.