The Rules of Survival by Nancy Werlin
For Matt and his sisters, life with their cruel, vicious mother is a day-to-day struggle for survival. One of Matt's early memories involves getting up during the night to sneak a cookie and being caught by his mother. Giggling and yelling "Cookie thief," she holds a knife to his throat, cutting him just a little bit to teach him not to steal. Unfortunately, the adults in Matt's life, a distant father and an uncaring aunt, don't help. Matt sees a glimmer of hope in Murdoch, who dates his mother, Nikki, and then leaves when he becomes another target for her escalating rage. It is Murdoch, with a violent past of his own, who is willing to risk getting involved and eventually becomes the change agent that the children so desperately need.