Help I am desperate! Please list details 4 help!!?

Alright here I go, My parents are divorced and have been since I was about three years old. As of right now I live with my father and his new wife (my stepmom) along with my half sister. My mother does not pay child support and at the moment I am being penalized for it. My step mom and my father do not let me have or do anything because they say "We do not have any money for you because, your mom does not give it to us," yet they pay nearly $600 dollers a month so that my half sister can go to a private school! I get blamed for everything and do everything for them and nothing is enough. I have to fight for the simpilest thing and everything is my fault since my mother does not pay child support. And like if he has a problem shouldn't he take that up with my mom, not me? Why is it I can't have a cell phone, a few dollers here and there to go to the movies, just because of her??? But my half sister does just cause she has both her parents in the house?? What should I do??