Don't Argue

Is someone is grumpy, they aren't necessarily thinking rationally. They are thinking grumpily. To point this out to them will only make them more grumpy. And you try to actually engage them in arguments, you won't win. They aren't going to listen to you, and they will get even more grumpy.

So don't get drawn into their arguments. If they say something, just say that you guys can talk about it later. Not "later when you are sane" :) Just "later on". If they push you for an opinion like "Wasn't your sister a Demon last night?" let it roll off with something non-committal like "I suppose so." You guys can always talk about it later when you are both in a better frame of mind. But to continue a fight when one person is already upset is a no-win situation.

Let the grumpy person run out of steam, and then talk about finding better ways to handle the issue in the future, to defuse the situation BEFORE the grumpy person gets that upset.