What should I do about my 16-year-old brother who verbally and physically abuses his parents, while my father does nothing but yell, and my mother prevents me from restraining him out of fear for him?

Your brother's actions towards his parents are forbidden and among the major sins, and he must repent. He should be advised gently and kindly. If your disapproval of his actions angers your parents, then express your disapproval to him away from them, for obedience to Allah takes precedence over obedience to parents if they contradict each other.