Alison Day-DiLaurentis was best friends with her twin sister, Courtney, and lived in Stamford, Connecticut. When they where in the 2nd and 3rd grades, Alison began to force Courtney to act as her doing terrible things. When their parents questioned Courtney, she said that Ali had forced her to do it. When Ali was questioned, she lied and said she didn't know anything about it. Courtney was sent to a mental hospital called the Radley, where she stayed until it closed down, when the girls were in the beginning of 6th grade. By that time, the Day-DiLaurentis family was simply, the DiLaurentis family of Rosewood, Pennsylvania. They supposedly had two children, Jason and Ali, because Courtney was a secret. The day her life changed, Courtney walked down to the kitchen wearing a hot pink shirt and indigo jeans, exactly what Ali was wearing. She started fighting with her and when she saw four girls in the yard, she went out to talk to them, discovering that they'd come to steal her Time Capsule piece, a piece of the Rosewood Day private school flag. She sat down, talking to them as if she were Ali. Her mother came out to the yard and said good bye to her. A few minutes later, Alison was being taken to the Preserve at Addison-Stevens, telling her mother she was Ali, and Courtney was outside. Her mother thought it was Courtney saying she was Ali, and she drove her to the new hospital. She became friends with Hanna, Spencer, Emily, and Aria, whom Ali was far from friends with. She ditched Ali's best friends, Naomi Zeigler and Riley Wolf, telling her new friends they'd had a fight and now they were her new friends. They became the most popular girls in the school, which was easy as Alison had already been the queen bee. At the end of 7th grade, the real Ali came back, the Preserve's doctors saying she was fine and ready to come home. Courtney's plan was falling apart.
Read more in Sara Shepard's novel, Pretty Little Liars: Ali's Pretty Little Lies.