A Streetcar Named Desire

Stanley's (Marlon Brando) cruelty towards his wife's sister Blanche (Vivien Leigh), culminating in a rape, drives Blanche to a nervous breakdown. Stella (Kim Hunter) and Stanley have Blanche commited to a mental institution. Stella takes her newborn son and leaves Stanley.
(In the eponymous Tennessee Williams play upon which the film is based, Stella stays with Stanley in the end.)

Thanks Taiga!