Influence question

I am looking for a book I read probably about 8 years ago, I do not remember the title,author, or what the cover looked like. All I remember is a vague description of the plot.
It was something like this: a story about a boy and his family from his perspective, the mother seemed to have a mental illness possibly and was slowly becoming increasingly erratic. The younger sister, barley old enough to quite understand what was going on. And him about age 14 I believe. The mother would date various mostly low quality men. Until one specific man she dated stuck around for perhaps a couple months. He was more focused on acting like a father figure to the two kids and they in turn began to accept this normalcy. However, at some point I believe this mans relationship with the mother wavered and he stopped coming around. One specific scene I remember is they were having a pinic at a park, something set the mother off and she grabbed her daughter and dangled her over the cliff ledge. I remember a vague passage about how the boy began lifting weights at school in order to (should the need arise) protect his sister from their mother.