If you read a lot of contemporary "issue" young adult, you're going to have this twist figured out 150 pages into the novel. Though the author makes some horrible choices in an attempt to throw you off the scent - things that don't really make sense in the real word. That's really where this book lost a star (lack of originality and choices that don't make sense just for shock factor).
Yet for some reason I found this book entertaining. It was definitely a good plane trip read because I could be immersed in it, but I could also put it down without any sadness or regret.
Overall, I rated it 4 stars because I found it entertaining, but it wasn't really something great, but it sure as hell wasn't something bad, either. It was just a middle of the ground sort of sister issue story.