I HATE my brothers girlfriend, advice?

So I'm 18 and my brother is 20 and he and his girlfriend have been going out for about 4 months. (she is his first girlfriend) He met her through me because she and I knew each other from work and one day while she was over my house smoking they got to know each other. Right when she started dating him, I knew there was something I did not like about her. She constantly needs his attention, she gets upset with him when he wants to hang out with his friends instead of her (which is like never).. she lived at our house for the first 3 months.. spent every single night here, laying in bed till 3 PM like a lazy slob, shed be there even if my brother wasn't just because she had no where else to go because she has no friends.. She doesn't clean up after herself. My mom and I are constantly picking up after her mess. My mom will feed her dinner then she'll just leave the plates sitting around everywhere and leaves like its her own god damn house. Shes icky, all they do is have and she and my brother leave condom wrappers all over the room and don't even worry about what my mom thinks. He has turned into just as dumb and disrespectful as she is and I can't stand it. My brother never used to be that way... He and I have gotten in more fights than we ever have because of this girl... and now I just found out that shes cheating on him most likely. Her and her ex have been hanging out behind my brothers back but the thing is I know he will just deny it and keep seeing her... I mean it is so ridiculous! He's losing all of his friends just because of this little rat and shes not even cute!! shes nasty... after my mom saw all of this she told my brothers girlfriend that she is not allowed to sleep over any more because it was just too much... But then last night she slept over again because she "accidentally" forgot to set an alarm... so this morning I couldn't take it anymore.. when my brother went to take a shower I went up to her and said "you know you're not allowed here right?" and she was like i didn't mean it bla bla.. and then my mom got mad at me for saying anything. my mom is way too nice! she lets her get away with everything... she told her "o its okay im sure it wasn't intentional" bull sh*t!! I know for damn sure she knew what she was doing. Now my brothers all pissed at me for saying something just ugh.... this is all sooooooooo pathetic