Will and Asheley (really didn't care about the spelling of her name) live with their alcoholic mother. Their dad left when Will was 6 and Asheley was 3, and while Asheley idolizes their absent dad, Will remembers what a horribly abusive person he was.
These two kids were so messed up, but... I don't know, it's weird, I didn't find the characters realistic. I'm telling you from experience, if you grow up with an alcoholic you don't celebrate their departure with drinks. And that's what these two did. After growing up watching their mum go into drunken rages, after their mother attacks them and leaves them broken on the floor crying, clutching each other - every night it's, "hey, have a margarita", "let's celebrate with some wine" etc. Just... no.
Then, when their mother is taken away by some creepy religious cult for "rehab" (what...), Will snaps and thinks he must protect his sister from everything, so she won't feel the pain he had to feel. And he also takes a weird turn into a somewhat incestuous attitude towards her. But more importantly, he snaps and starts killing a bunch of people he sees as a threat to her.
There's this big reveal at the end of the book about how Asheley was manipulating her brother, I don't see how that's surprising? I mean, it wasn't to me, it was pretty obvious all along that she was manipulative, Will did every thing she ever asked, even dressed the way she wanted. And the answers she gave were all too, "aww chucks, I love Will and I want to protect him, and I feel so guilty, but he's totally a violent murderer and I didn't have any control over the situation". It's like... wow, what suspiciously specific denials, Asheley.
Anyway, this whole story is told in alternating POVs, Will and Asheley's, while they're at a police station in Mexico. And to be perfectly honest, I started feeling sorry for the cops who had to listen to all the 6572406756 mundane, irrelevant teen drama details surrounding all the murders.