Which 1960s "Rat Pack" celebrity was the brother-in-law of John F. Kennedy?

The 1960s "Rat Pack" celebrity who was the brother-in-law of John F. Kennedy was Peter Lawford (1923 - 1984). He was often called the least well-known member of the Frank Sinatra “Rat Pack" because he tried to keep a very low profile in news media.

Most fans, critics, and others most often remember Lawford as a suave and handsome British movie star. During his acting career, he performed in films which include “Little Women” (1949 family drama), “Advise And Consent” (1962 political drama), “Exodus” (1960 epic drama), and “It Should Happen to You”(1954 romantic comedy).

In all but one or two of his films, Lawford was a good supporting actor. He was never truly a leading man. Some movie critics of the 1950s and 1960s reported that if it wasn’t for his involvement in the "Rat Pack" and his connection to John F. Kennedy, he would be best known as a British-American socialite. it was said that he was "famous for being famous".