Emotional abusive stepfather?

My childhood was plagued with physical/emotional abuse, my adolescence was plague with sexual harrassment/more emotional abuse, and now, as a 20 year old, still living at home when I'm not away at school, I am plagued with nothing but the most horrendous emotional abuse. He calls me names, shouts at me, gets in my face and calls me a *****, c**t, etc, he hits my sister, who is mentally retarded, and forces her to do way more chores than she should have to (which have to be done perfectly), he gets angry when I leave to go see friends and my boyfriend, and my mother stands by and does nothing. I'm afraid to call the police because I am afraid they will not believe me. I don't have the financial resources to get out just yet, and my stepfather won't let me have the car that my grandmother told me I could have (she died before her will was completed). He makes me pay him $175 per month to live here in the summer, which is everything I have left over after paying for my cell and loans. I really don't know what to do here anymore. I would go stay with a friend but I do not want to be a burden, and as much as I hate that man, I don't want to leave my mother and sister. Is there anything the police can do? Or is it too late?