Something really, terribly awful.

Last night my brother got sexually harassed by my new step father. Here's what happened:
I was at home, (yes i still live with my mom. I've got to finish school still.) in my room playing final fantasy 13. The next thing I knew I heard my brothers door slam shut. (His room is next to mine.) I go to see what's wrong and he's crying. It turns out our step dad felt him up.i didn't belive him until later that night.
After dinner I went into my brothers room to check on him and sure enough there was my step dad.
Now I won't go into details, though I will say i kicked the bastards ass until my mom came in. We told her what her new bastard of a husband did, but she doesn't beleive us.
What should I do? I don't want my brother to be mistreated. I mean for heavens sake he's pretty fragile. My mom won't listen. I swear if he hurts my brother again I'll kill him.
Advise please.