Can't stop thinking about it?

About ten months ago I read this book from my school library which my friend had recommended (I cant remember the name or author), I read it and my thoughts for the entire book were, this is awful and kinda twisted. I couldnt understand how anyone liked it. The story followed a girl as she grew up, from a very young girl to an adult. Her dad had an affair with her mums old best friend and the parents eventually spilt and the other women got pregnant and the father took off somewhere. The other woman already had a daughter the same age as the main girl who at this stage was about six, the daughter was weird and a bit sadistic which the main girl thought was kinda cool, but she also hated her. As the main girl grew up she realised/decided she was a lesbian and did some pretty weird things in the process, her mother got depressed and became distant. The book had no clear ending.

Chances are none of you have read it because it was the only book by a small Australian author. But my point is that since reading it every so often I catch myself wondering about this book, why was the main girl so odd? What was her father thinking? What happened to the other womans daughter who was always a little bit strange but ended up caring for her half brother who was the child of the affair?

So my question is have you ever read a book, which you didnt really like but you feel like you will always remember it?