Little Monsters...

This book is all around messed up! See if you can find out who the psychopath is before page 322.

From Goodreads.com:
For fans of Pretty Little Liars, Little Monsters is a new psychological thriller, from the author of The Darkest Corners, about appearances versus reality and the power of manipulation amongst teenage girls.

Kacey is the new girl in Broken Falls. When she moved in with her father, she stepped into a brand-new life. A life with a stepbrother, a stepmother, and strangest of all, an adoring younger half sister.

Kacey’s new life is eerily charming compared with the wild highs and lows of the old one she lived with her volatile mother. And everyone is so nice in Broken Falls—she’s even been welcomed into a tight new circle of friends. Bailey and Jade invite her to do everything with them.
Which is why it’s so odd when they start acting distant. And when they don’t invite her to the biggest party of the year, it doesn’t exactly feel like an accident.

But Kacey will never be able to ask, because Bailey never makes it home from that party. Suddenly, Broken Falls doesn’t seem so welcoming after all—especially once everyone starts looking to the new girl for answers.

Kacey is about to learn some very important lessons: Sometimes appearances can be deceiving. Sometimes when you’re the new girl, you shouldn’t trust anyone.