Dealing with a paranoid schizo

My boyfriend and his 2 brothers moved to my city about 2 years ago.. They don't have anyone here .. My boyfriend and I now live together and his brothers live with us. But one of them is loosing his mind, like literally. We used to get along very well , and even would call me "sister" I don't know what happened, he became very nit picky about everything I do. He became very paranoid, he started saying that people at his work were conspiring against him to kill him, and even told my boyfriend I was in on it. And that I was a spy..so he quit his job and now just sits at home coming up with crazier and crazier things. They argue a lot and even pulled out a knife on the other brother during an argument. Now my son lives with us too. And i no longer feel safe with this crazy guy around. I already told my boyfriend I don't want his brother with us, but he told me he can't send him back home. Something about his mom being too sick to deal with him. I don't know what to do, I'm really scared of him. I feel like he's a ticking time bomb.