Patricia Helen "Pat" Kennedy Lawford (May 6, 1924 – September 17, 2006) was an American socialite and the sixth of nine children of Rose and Joseph P. Kennedy Sr. She was a sister of President John F. Kennedy, Senator Robert F. Kennedy, and Senator Ted Kennedy.
 Pat wanted to be a film producer, a profession not readily open to young women in her time.
 She married English actor Peter Lawford in 1954, but they experienced a serious culture-clash and divorced in 1966.
Lynn Franklin (born 1922 in Mississippi) is an American author, and the most highly decorated officer in Beverly Hills Police history.
 He is most famous for being the Beverly Hills Police detective who pulled over a car containing Robert F. Kennedy, Peter Lawford, and Dr. Ralph Greenson (Lawford was behind the wheel), the night he claims Marilyn Monroe was killed.
 He received the Clinton H. Anderson Award.
Joseph Patrick "Joe" Kennedy Sr. (September 6, 1888 – November 18, 1969) was an American businessman, investor, and politician known for his high-profile positions in United States politics.
 Kennedy was married to Rose Kennedy.
 Three of their nine children attained distinguished political positions: President John F. Kennedy (1917–1963), Attorney General and Senator Robert F. Kennedy (1925–1968), and longtime Senator Edward M. "Ted" Kennedy (1932–2009).
 He was a leading member of the Democratic Party and of the Irish Catholic community.
 He was appointed by President Franklin D. Roosevelt to be the first chairman of the U.S. Securities and Exchange Commission (SEC), and later directed the Maritime Commission.
 Kennedy served as the United States Ambassador to the United Kingdom from 1938 until late 1940, when he annoyed Roosevelt by his pessimism about Britain's survival.
Christopher Kennedy "Chris" Lawford (born March 29, 1955) is an American author, actor, and activist.
 He is a member of the prominent Kennedy family, and son of actor Peter Lawford.
The Rat Pack is a term used by the media to refer to an informal group of entertainers centered on the Las Vegas casino scene.
 Having its origins in a group of friends that met at the Los Angeles home of Humphrey Bogart and Lauren Bacall, by the 1960s, it was the name used by the press and the general public to refer to a later variation of the group that called itself "the Summit" or "the Clan", featuring Frank Sinatra, Dean Martin, Sammy Davis Jr., Peter Lawford and Joey Bishop among others; they appeared together on stage and in films in the early 1960s, including the movies "Ocean's 11", "Sergeants 3", and "Robin and the 7 Hoods" (in the last film, Bing Crosby replaced Lawford).
 Sinatra, Martin, and Davis were regarded as the group's lead members.
Lieutenant-General Sir Sydney Turing Barlow Lawford, KCB (16 November 1865 – 15 February 1953) was a British Army officer, and the father of movie star Peter Lawford.
Edward Moore "Ted" Kennedy (February 22, 1932 – August 25, 2009) was an American politician and lawyer who served as a United States Senator from Massachusetts for over forty years from 1962 until his death in 2009.
 A member of the Democratic Party, he was the second most senior member of the Senate when he died and was the fourth-longest-continuously-serving senator in United States history, having served there for almost 47 years.
 Ted Kennedy was the most prominent living member of the Kennedy family for many years, and he was also the last surviving, longest-living, and youngest son of Joseph P. Kennedy, Sr. and Rose Kennedy.
 He was the youngest brother of John F. Kennedy, the 35th President of the United States and Senator Robert F. Kennedy, both victims of assassination, and the father of Congressman Patrick J. Kennedy.
Patrick Kennedy (February 16, 1823 – November 22, 1858) was an Irish farmer, businessman, and politician who moved to East Boston, Massachusetts from County Wexford, Ireland.
 He was born in New Ross, Ireland.
 He was the father of businessman/politician P. J. Kennedy, paternal grandfather of businessman/politician Joseph P. Kennedy Sr., and patrilineal great-grandfather of World War II casualty Joseph P. Kennedy Jr., President John F. Kennedy, Senator Robert F. Kennedy, and longtime Senator Ted Kennedy.
Betty Lawford (February 1, 1912 – November 20, 1960) was a United States-based English film and stage actress.
 She was briefly married to the American actor-director Monta Bell.
 She was a cousin of the actor and socialite Peter Lawford.
 She died at Roosevelt Hospital, Manhattan, following an illness of three weeks.
 Her parents, Ernest Lawford and Janet Slater Lawford, were also actors.
Sergeants 3 is a 1962 film directed by John Sturges and featuring Rat Pack icons Frank Sinatra, Dean Martin, Sammy Davis, Jr., Peter Lawford, and Joey Bishop.
 It was the last film to feature all five members of the Rat Pack due to Sinatra's falling out with Lawford.

Question: What sister of a longtime Senator from Massachusetts married Peter Lawford?
Answer: Patricia Helen "Pat" Kennedy Lawford