One of the reasons that Hannah was able to manipulate Sara the first time was because Sara identified with her. Both are very smart and since Sara could see her intellectual self in Hannah at that age, she could believe that Hannah could do those things. So instead of discounting Hannah, like Nick did, Sara saw the possibility. Unfortunately, she was wrong. I think that is what makes Sara so susceptible to Hannah's machinations, that feeling that Sara knows her, that overly smart kid and what it felt like. However she now sees an evil twin looking back instead. I don't know if this makes any sense at all , but I hope you'll get the gist of what I mean.