Alison has a twin sister named Courtney. Courtney has a history of pretending to be Ali since she has a mental issue, and she stays at a hospital which is why we never hear of Courtney. So one day (the day the girls try to steal Ali's flag), Courtney is home because she is switching hospitals, and Ali is up in her room. Courtney comes and befriends Aria, Spencer, Emily and Hanna while Alison is in the house. Mrs. Dilaurentis mistakes Ali for Courtney and takes her to the hospital. When Ali tries to tell her mom she is Ali, her mom thinks it's just Courtney pretending to be Ali again. So, Ali then spends some time in the preserve that Hanna stays at in the series which is how Iris knows Ali who she thinks is Courtney. So, this whole time Courtney is friends with the 4 girls who they think is Alison but it's not.
Then at their sleepover, Courtney who is really Ali comes back from the hospital for summer. She is jealous of Courtney's life and how Courtney took it from her. Meanwhile, Courtney (as Ali) is with the girls having their 7th grade sleepover. When Courtney (as Ali) tries to hypnotize them, Spencer gets mad and orders Courtney (as Ali) to get out. Courtney goes outside to meet up with Ian after that, but in the act, Alison (as Courtney) takes her and pushes her in the hole, killing her. Btw, Alison was the one who was stalking them in the window during the sleepover.
Then, when Courtney (who everyone thought was Ali) doesn't return, the family and her friends thinks Alison is missing, but really the REAL Alison who was never friends with the 4 girls as they thought, is still at the preserve. Then in Wanted, Alison (as Courtney) returns and the girls believe all her lies as she tells them she is Alison (who they never really knew...they knew Courtney). Then Alison tells her story acting as if she was their best friend.
On their trip with Alison (acting as their old best friend who was Courtney but they thought she...