Courtney and ali (Spoilers)
Ok so i know that courtney killed ali , BUT in the books how did everyone finally figure out that ali was still alive ??
Alison killed her twin sister Courtney. Courtney, from my understanding is the crazy insane sister who was supposed to go to the mental hospital. Instead, she managed to switch herself out with her sister Alison. Courtney went on living as "Alison" and made friends with the four girls: Hanna, Spencer, Aria, and Emily. She then wrote a diary telling all the secrets of her four new friends. Then, the real Alison broke out of the mental hospital, found her twin sister, Courtney and killed her.
The A-team is formed from the people who hated "alison" aka Courtney because she was a crazy psycho-bitch. Alison being the normal one out of anger is now tormenting her friends and is somehow omipresent.