I have a theory that Sara,Bethany and Hanna are all sisters! Bethany and Hanna are identical twins to Scott Montgomery( Harold Crane) and they had a younger sister called Sara ( one year younger than Bethany and Hanna). Bethany and Sara were the girls in Alison's story and the doll they were fighting over was Hanna! Bethany and Sara were always competitive with each other and wanted what the other wanted! Scott was always mentally ill and couldn't handle raising three girls after his wife died! When a social worker came to the house one day she seen this and the result of the fight between Bethany and Sara (PS when we seen the twin getting stabbed in the story, it was actually the doll (Hanna) who was stabbed in a raged by Bethany). Scott was then admitted to Radley along with his persued mentally ill daughters and a traumatized Hanna. A few months later and Hanna was adopted by the Marins, blanking out the horror that happened to her as a child and now believing the Marins were her real family! Then a few years later, Sara was adopted, which made Bethany very bitter inside and turned! When Harold got out of Radley, he didn't want to have contact with his violent daughter Bethany and wasn't allowed to see his other daughters as they were already adopted! This made Bethany, not only loth Sara Harvey, but Hanna and Scott as well! Scott soon became a recluse and opened up the motel, but Bethany never forgot! She befriended Charles (later Charlotte) , but Charlotte became friends with a new penpal, Sara, and Bethany framed Charlotte for Marion's murder in spite! Then Bethany had a greater plan! Resenting Radley, she needed a permanent way out of the hospital and she became friends with Ali to do so! That Night, Bethany had a mission, to kill Mrs. D and blame Hanna for it! Bethany would disguise herself as Charlotte and when the police came looking for a young woman they would find Hanna! However, Charlotte thought that Bethany was out to get revenge on Mrs Dilaurentis (who drove their mother in suicide after Scott's affair with Jessica) and contacted Sara! Then Bethany and Sara had an altercation outside the barn and Hanna heard them! When Hanna arrived outside, she seen Bethany hit Sara over the head with a shovel and she let out a scream due to the attack, but also seeing her double ( the one Spencer heard when she was wondering) . Bethany knocked out Hanna and swapped clothes, making her way to attack Jessica! When Hanna woke up, she was disorientated and wondered to the Dilaurentis' backyard, where Mona hit her! Then Melissa buried her! Bethany witnessed all of this and couldn't believe her plan worked! She was free! Her and Hanna were so identical that people would assume that Bethany was dead and that Hanna was alive and well! It was better than her original plan and there was no blood on her hands! She went back to the barn that night and fell asleep! That would explain Hanna's drastic change when she hung around with Mona! It would explain why Hanna wanted to get Mona out of Radley! Bethany owed Mona for her freedom! Also when Sara woke up,she knew what Hanna! She blamed Bethany and knew she was alive! It would support the fact that's why Sara saved the other liars and not Hanna (Bethany)during the lodge fire! Sara and Bethany's game is dark! Maybe when Sara told Emily that she tried to keep her safe it was from Bethany by spending time with her and keeping her from the liars!
^ Actually I think they're the same age. Because remember, in 6A, Sara was trying to get emancipated, as she was under 18 and her mother was a drunk. Of course, we don't know if she was really lying about her mother being so horrible, but I'm pretty sure she's the same age as the liars.
Sara is the same age as the liars, because when Hanna had the idea to search for other girls who are as old as Alison and gone missing at the same time they found out about Sara, so she has to be the same age. Additionally, the liars met some friends of Sara who were definitly the same age.