"The Rules of Survival" by Nancy Werlin

But how come there don't seem to be any rules about when you ought to help others survive? Rules telling you when that's worth some risk to yourself? Callie and I were working so hard for you, Emmy, but as far as I could see, nobody else cared at all. For any of us.

Matthew, Callie, and their half sister Emmy, live in a world of fear. They are ruled by it, controlled by it, they live their life by it. It's what keeps them safe, most of the time.
Safe from their mother.
Safe from Nikki.
Nikki, their mother, has issues. She only comes home at two or three in the morning, often drunk. She brings home men, she swears. She's obsessive about her children's love- they must love her, and only her, or they will pay. They will be sorry. She will take her revenge.
It's not the only think that Nikki can be obsessive about, Matthew and his siblings learn.
When Matthew and Callie track down a mysterious Murdoch, that Matthew saw in a store, their mother takes control. Within weeks, Murdoch and Nikki are dating, and life seems amazing for the three children.
Until it happens.
When Nikki goes psycho in front of Murdoch, he dumps her, and the children's lives are thrown in to chaos. It's back to the old mother, but worse.
And when Nikki starts obsessing over Murdoch, Matthew knows they have to get away.
Fast.

What we knew was that her homecoming was always the cue for a play- an elaborate production of live theater.
I was the director of our theater, arranging the stage set, telling you and Callie to take your places, prompting you to do or say this or that, whisper-feeding you lines of dialogue and bits of business. "Don't forget to hug her!" "Go get her some Advil and a glass of water, fast." "Ask her if she'll help you with your homework later, she likes that." "Stop stomping around, she'll go ballistic."

I really liked this book, for a couple of reasons.

First of all, I really liked the characters. Matthew, the narrator, was interesting. He had his own realistic, normal, yet very heroic, mind set. I could identify with him, yet at the same time I sort of looked up to him as a role model. I like main characters like that.

The book was written in the form of a very, very long letter from Matthew to his half sister, Emmy. As such, it has some detours, footnotes from Matthew, and it's written in the sort of way that you would expect a letter to be written. He's talking to Emmy, and it shows. I like that. It doesn't feel unrealistic, like "let's write a letter... now it's going to be told from third person POV and I'm only going to say it's a letter at the beginning and end."

Third of all... Nikki was evil. I found myself completely living in fear of her myself, at least while I was reading the book. She was... unpredictable, and I think the author did a very good job of making her act in the way you expected her to, as Nikki.

All in all, a very excellent book.