Should I be worried about tomorrow?

A few years ago, 3-4 years ago. My older brother left home after an argument with my mother, he hit my mother and threatened her, and he recently got in contact, and he is coming to visit tomorrow, and I don't trust him one bit, he's a self harmer, if he's not afraid for his own life, what about his families? What if he kills my mother and sister right infront of me. My mum says. "Don't worry, if he does anything, phone the police." The police take 10 minutes at the minimum!!! What should I do :( ?? I don't want to lose my mum.