Dan, are you one of those psychic people? I know a few of those..They were alwasy so fat and stinky...eugh....
Anyway, I grew up with a cousin like that. He'd always go off at any minute for no reason, I could always tell because he'd bite his upper lip...and just go berzerk for no reason. I remember one time he and my sister were on the trampoline, he randomely came charging at her and just threw her off the trampoline, she hit the cement and scrapped up her face. It was pretty bad...
for some reason I was the only person he ever listened to though.
He was later diagnosed with some kind of schitzofrenia....
I think, you should sit down and talk with your parents about it, then I guess have a family talk, unless your parents only want to talk to him. I'd say try asking him nicely what bothers him, just like outsiderzedge said. It's never been tried, right? It's worth a try...and if that doesn't work, then there's really nothing you can do except maybe move out. I suppose you could try social services..But, I don't think the situations that bad for them to do anything.
Of course, if I were the parents, I'd tell him to get help, or kick him the **** out of the house. From the way you make it sounds, if the parents keep babying him, he'll never have any incentive to change. Or if he has some mental problem, it won't get helped the way things are going..
Or maybe you could try going to a counselor yourself, and asking for ideas on how to deal with him?