My step dad abuses me, what should I do?

My step dad scares me and my older brother the most out of the 4 kids in the family. The 2 younger ones of course his own flesh and blood, don't get hurt yet we do. Ok so you know at a young age you mess around and have fun. But when me and my older messed around, we would be strangled to near death, or grabbed by the shirt and banged against the wall. Our mother would just stand there watching us dieing slowly, and wouldn't do anything. When the 2 younger kids got abused my mother would defend them, and yet I always wonder how come she doesn't defend us. (I would mostly get beat up the most because the family blames everything on me for some reason. Also I'm kinda the middle child.) Everyday I used to hide from my step dad or avoid him as much as possible when he came back from work. Sometimes when he strangled me, I wished he strangled me so hard that I would die and move on to the next world. When ever I did got strangled I had bruises on my neck, the next day I would go to the nurse in school. She would always ask "what happened to your neck, it looks like you were strangled", I afraid, would make up a lie to her every time. I was afraid what would happen to me, what would happen to the family if he was put in jail. So to present day now, I only get child labor time to time, and get get or yelled at.