Have you ever asked your parents why they sit idly by while their younger son is becoming a monster? To be blunt, this is idiotic parenting. There's not enough info here to guess whether he's mentally ill or simply a spoiled brat who never outgrew toddler tantrums, but in a way it doesn't matter. Your parents are the only ones who can fix this and, if they choose not to, what happens next is on them.
Obviously, your age helps here, because if you consider all possible options, there's probably a way you can get out of that home fairly soon. Until then, tell your parents if he is becoming violent, you WILL defend yourself. This might mean physically defending yourself or it might mean calling 911. Whatever seems best in the moment.