Huge ordeal going on, REALLY need advice here.
I've became close to this guy whose really cool and works a lot so he's stressed enough as it is (C). Problem is, his barely younger brother (F) is a prick-of-all-pricks. I mean, holy shit. Never seen something like this. He seemed chill at first, but lately his true colors have came out. I won't go into detail because it's too much, but it's gotten to the point where I am not sure whether to be worried he might try to stab me if he gets pissed enough, if I should just keep standing up for myself when he calls me names or tries to take "jabs" at me (calls me cunt, etc) by telling him to fuck off, tell his brother to discuss with his family about seriously doing something to get him mental-related help, ignore it all and just try to stay calm and make C happy, or just be a psycho bitch around him and treat him like shit, or take petty "revenge" on him.
this prick is abusive to his gf apparently too, no one does shit about it.
C and F got into a brawl last night (they are both adults, mind you) and C was so stressed and angry he started to fucking cry a bit.

So, I don't want to just let this slide. What are your opinions? Maybe I'm not in a regulated state of mind because I am upset, so if you guys have suggestions. Thank you.

tl;dr Crazy, psychotic brother with anger issues of the guy i'm dating needs to fuck off and i don't want to just let it slide and let things get ruined due to it or let me or C get stressed because of it. What do?