I'm going to cut off my relationship with my step sister
Why do you want to cut your step sister off?
I want to do things my way and I'm not going to let her get in the way of my life. In fact, I don't want anyone to get in the way of my life.
I can understand that you want things done your own way. People should not sever relationships with family without good reason. You should explain to her while you are feeling the way that you do.
Nah I just don't want her and that's the end of it.
I still think you should tell her why you are cutting her off, to avoid conflict.