1. document everything u see and hear.
2. try to even get some of it tape recorded if possible without her knowing.
3. keep reassuring your step brother that he does not deserve that crap and tell him it is abuse.
4. talk to someone at his school if u possibly can without jeopardizing ur situation in the home so they will be aware of the situation and hopefully give him extra emotional support.
5. teach him what invalidation means. if u dont know, google it. my site will probably be number one but u can read other stuff too
6. keep writing me for emotional support
7. try to find some other people to give u emotional support. i am afraid u might be taking on a lot without much support urself.
8. dont confront the abusive person directly for now.
9 try to show her understanding and give her emotional support and maybe try very subtly to get her to express her feelings to u but i am afraid if u show any signs u are trying to "stick your nose in things" she will get defensive and hostile. how is ur relationship with her so far?

well those are a few for now. ur role is very important in his mental health. each abused person needs one adult who reassures the abused person that they are not the ones who are bad, wrong, naughty, lazy, etc etc. and that the problem is the unhappiness, anger, resentment, unmet emotional needs and lack of skills of the abuser.

best wishes

steve