Need help getting rid of my boyfriends psycho deranged brother?

I stay at my boyfriends parents most of the time, also living here is his grandmother and his brother. His brother is severely bi-polar and schizophrenic and has attacked people in the house hold numerous times, so far he has physically attacked his mother, father, and my boyfriend/his brother. he attacked my boyfriend the other day and was arrested and kicked out for like the 20th time and his parents continue to let him live here because no one else will take him in and he refuses to get a job and his own place, also he has a son that i am extremely worried about being raised by such a nut. But anyway, his parents continue to let him stay here even though they know that every time the fighting gets worse and more dangerous, to the point that i fear for peoples lives. Since im not family i pretty much have to keep my mouth shut and deal with it but no matter what my boyfriend says to his parents about how extremely messed up it is that they let him stay here, especially knowing that it WILL happen again and be much worse the next time. but they ignore how serious the situation is only cause they don't want him on the streets. Not only is he dangerous but he is the most unpleasant person to live with, he constantly blasts metal throughout the house, he slams everything and stomps around, talks way too loud, takes whatever he wants because he thinks everything belongs to him including my boyfriends car, and leaves disgusting messes everywhere without cleaning up after himself. but if you confront him about any of it he with start a ridiculous inappropriate fight and eventually turn it physical, and almost every time goes for a weapon, for example the last fight between him and my boyfriend was only two days ago and he threw a chair at him without warning. He also is now starting not to only threaten to hurt his mother father and brother but has added me to his list too starting arguments with me just because he used to date my sister and he knows i hate him for the sick horrible things he did to her and destroyed her life. He has no remorse for the evil things he has done, and it is very obvious that to him he is just proving to everyone that he is the one in control, especially since his parents let this **** keep up. this dangerous nightmare needs to go before someone gets seriously injured or killed. we have been doing everything we can to try and get jobs and get out but its proving difficult and taking too much time, we need a quick solution to get rid of this monster. i have no idea what to do, i feel so helpless and so does my boyfriend.