Out of control brother age 20, need immediate help?

My brother has become out of control and abusive since dating his girlfriend. He throws things, screams, hits me and swings at our mother, lies, steals, and more. He overdrafts his bank account on a weekly basis by hundreds of dollars, and mom is passive and does not hold him accountable for anything. The girlfriend is a huge problem-we found a blog she kept for months about what idiots our family is, and she deleted it-but it's still a nightmare dealing with them. Now, the new thing is a 4th of July trip he wants to go on with her. My mom wants to kick him out if he goes, but I think it is going to kill her. The situation would be easier to deal with if my mom didn't have stage 4 breast cancer. The stress of my brother's actions are too much on her. The situation would also be easier if my dad was around, but he is just as dysfunctional as my brother, and my mom is currently filing for divorce. I let them move into my home a few months ago, and I wish things were easier. Any advice? We need help.