My brother just left the house after a fight. What can i do?

My mother has been angry at him because his girlfriend only comes to our house when there is nobody except for me(i'm 13). They stay at the living room a while before they go to his room for hours and later come out. She is at my house usually at 7:00 a.m until 6:00 p.m or something around that. Then they leave and my brother comes home around 4:00 a.m. My mom is angry because they are giving me a bad example. My mother came home today to eat lunch with me and the girl was here. My mother politely told her what she tought and my brother started arguing with her and left with his girlfriend. The last thing he said was "Don't even call me." and closed the front door. My mother was under lots of stress after that and i thought she was going to have an stroke. After she left i started crying because i don't know if i will ever see my brother again. This has been going on for about a month. He is 21 years old and dresses like a punk and does not shaves or do anything to his messy unruly hair. He looks kinda like Hagrid(the giant form harry potter). I'm so confused if he doesn't come back i'll go running through town looking for him. I've never had that of good relationship with him but now that he left i miss him more than anything. What can i do?