I'm scarred for my own life.

My brother (15) has threatened me multiple times that he will kill me and two time he has threatened to kill me while he was holding knifes. I have request my mum to take him to the hospital and get checked by he mental health team and all I get back is the same phrase over and over again "it will hurt his chances of getting into the military" my older sister (19) says "we all would have to bring in our therapist and It won't go anywhere" they aren't the ones being threatened by him, I can't move away from home as my mum is my career. Yet I have been told if I go to the police I will be thrown out on the streets. what the hell am I meant to do?I don't want to and I know I won't be able to cope being thrown out