This is my theory on Jessica and Mary Drake.
NOTE: This is a THEORY let me know what you guys think..
Obviously the twin theory also exists in the show (like in the books only it's not Alison/Courtney) it's Jessica and Mary.
1. So Mary told Spencer that Jessica was jealous of her (like Alison was of Courtney) they never got along, and always fought. Mary was in Radley when Spencer was born. (Possibly Charles/Charlotte/CeCe as well I can't remember) In the books, Alison had a mental disability but made everyone think it was Courtney. Who is to say Jessica didn't do that as well to get rid of Mary?
2. Mary was talking to Doctor Rollins. She made the comment "Finally taking back what was supposed to be mine." Did Jessica steal Mary's life? (Or Vise-Versa?)
3. Jessica also told Alison the Hastings (including Spencer) were not to be trusted. (Did Jessica know Spencer was Mary's daughter, and if so, why say this if you hate your sister yet you were willing to raise her first born?)
4. Jessica turned everyone against Mary (like Alison/Courtney in the books)
5. There was also no mention of Mary for 6 seasons.
6. Remember the twin story Ali told the 2 boys when Hanna was babysitting? Could it be a reference to Jessica/Mary? (I'll explain further down)
7. Remember when Jessica said "That wasn't my daughter in the chair, it was CeCe Drake?" (Note how did she not know CeCe/Charlotte were the same? Why did she sound so coldly saying it? Did she know CeCe was Mary's all along?
8. What if Mary is really Jessica? What if Jessica actually killed Mary and stole her sister's identity to ruin her life?
Do you guys agree with this theory or not?