Because I think she's pretty clearly autistic. Obsession with time, order and routine is very much an autistic trait. A lot of autistic people really can't cope with changes to schedules, and have to do the same things at the same time of day, or the same place etc.
Also, she can't distinguish between lies (ie from her father) and reality, and she has an obsession with moths and clocks, a very good memory (lots of autistic people have photographic memories), she doesn't understand other people and their emotions, and a general inability to empathise are all autistic traits. She's very precise in the way she describes things, and doesn't understand how people don't necessarily share the same interests as her.
The doctor spends a lot of time with her as a child, encouraging her to talk to him. And her mother accuses her of ruining her life. She only gets accepted into the school because her sister does, and she can't make friends with the other children there.
Plus there's the policeman's reaction (he seems shocked when he realises which sister she is), and the fact that there's a feeling that she's being taken advantage of with the whole pregnancy thing.
Plus there's a lot of references to "retards", which I see as hints.
I might be wrong, but that was my interpretation of it. I do quite a lot of volunteering with autistic kids/teenagers, so that might be why I see it like that.
Edit: sorry for the essay