Jessica DiLaurentis (Book Character)

Jessica DiLaurentis announces to Rosewood Alison and Jason have another sister, Courtney, who is also Alison's twin sister. In another twist, it is later revealed that Spencer's dad, Peter Hastings, is the father of Courtney and Ali. Courtney and Allison both know, and they assume that Jason and Mr. DiLaurentis know, considering how much the couple fight. Fake Courtney/actual Allison said that she thought they would have gotten a divorce by that time, but with all that had happened with Allison they decided to stay together.