What do I do when my step father remarries and his new wife will not allow me or my family to see my sister?

Okay my mom died 6 years ago. My step dad remarried two years later and he put me out of the house and moved in with her. His new wife is bitter and ignorant. For the last couple of years he has been neglecting my sister from everyone in my family except me. Since he has married this woman every time I want to see my sister I have to go through her. She is always saying something smart every time I call and after 4 years I got sick and tired and I fussed her out. Now she is telling me to never call or come to their house. So I cannot see my sister, and my step father is never anywhere to be found so its not like I can talk to him. Everything I want to do with my sister involves her. She will not allow any of my family to see her and she will not allow her to call us. She is always getting smart with my family members. Even when I was able to talk to my sister she(new wife) was always around listening. I am in college and I live with my aunt but my family wants to take my sister in. With all of the stress I get from college work, I have to deal with this. Everyday I want to cry and just do some things that I may regret in the future. It is so hard, I do not know what to do. I don't know when the next time I will even hear or see from my sister and I do not want her growing up thinking me or my family do not care about her. I want to be in her life. She hates her step mom and she hates living there. Also, her dad is never around. I feel so bad and stressed because I feel Like it is nothing that I can do. My step dad family does not even like his new wife, he also neglects some of his family from seeing her. She has like brainwashed my step dad because before her, I would have never thought he would allow these things to happen. Please, I need help before I do something I may regret because I am tired of acting civilized and getting nothing out of it.