Brother/Sister by Sean Olin

Will and Asheley have a troubled past. Their father left them when they were little, and their mother has just been carted off to an alcohol treatment center. Now, they have the house to themselves, and an endless California summer stretching out before them. Through alternating perspectives, they tell the story of how and why their lives spun violently out of control - right up to the impossibly shocking conclusion you'll have to read for yourself to believe.