I agree with Dani—this is a symptom of great unhappiness. Whether it’s medically depression or just being generally unhappy, that’s what it is.
As a sibling, I’d be more worried than angry. (And since I’d be angry, that tells you how worried I’d be.)
I’d be asking DH to call Ned and say, “You’ve been really angry and yelling at people, and it’s really, really unpleasant on the receiving end of it. Are you OK? What’s up? Are you and Carol OK, or how’s work, money, what?
“Can you find someone to talk to about this? I hate to see you so unhappy. Sometimes we can’t fix the things that make us unhappy, but we can find ways to cope, either logistically or emotionally.
“And my dearest brother, we NEED you to find a way to communicate that isn’t this rude and this unpleasant. On this holiday, we need you to be polite. I’m not going to sit quietly by and let you create all this unpleasant tension. I’ll take you out for a walk or something to help you cope, but if you won’t take the hint, we’re going to send you home.
“I’m sorry it’s come to this—I worry about you a lot. But I can’t let you turn Christmas in my home into an unpleasant gathering full of Ned yelling at people and farting in the living room. Capiche?”