Dear Alex:
I understand your concern. As I wrote earlier, it is important that you protect yourself from any more abuse, that you make yourself unavailable for more. If it takes no contact, make it so, is my suggestion.
If you are indeed close with your stepbrother, if he is close with you, the two of you will be able to remain close regardless of your mother input to him, I am thinking. The two of you can agree to not talk about her and continue your closeness.
You mentioned seeing a therapist. I hope you do. It is difficult to start adulthood when the adults in your life so far failed you. And they failed you. Yet healing is possible for you and your motivation to get better is evident.
Post again anytime you would like.
anita