It's a 50/50 Thing by Chris Higgins

Kally has moved to a new town, where she meets Jem, also a new boy, who, like Kally, seems to have some sort of secret. Quite a dark book at times. Jem seems to be devoted to Kally, and her little sister, but he is not so loving towards others. And where is Kally's dad? An older teen book for a change! And a good one.