Courtney was Alison's twin sister. Alison tricked everyone into believing that Courtney was insane because she was jealous of her more popular sister. It worked and Courtney was falsely diagnosed with Paranoid Schizophrenia. Courtney was sent to the mental hospital the Radley for three years. When Courtney was supposed to be switched to the Preserve (another mental hospital) she tricked their mother Jessica into taking Alison instead, which drove Alison to kill Courtney a year and a half later.~J
Ps. This is for the books not the Tv show.