imho the fact that the subject has said "horrific" things about OP and his family, would it be wrong for OP to simply not have contact with this person again? Is it a brother in-law, and if so, so what? Does he somehow get a free pass where everybody sees what the subject does, and they defend him? I can imagine if all of the above is true, what makes it difficult is it's a relative. But, still, as a brother in-law, I would say that is not necessarily the person who would be responsible for correcting the behavior. If a brother, that's totally different.
At any rate I do empathize. In my life, the only drama is provided by my wife's side of the family. Mine is too small, and, not "that" close where we would have conflict and feuds, etc.
I just can't help but feel that no problem would give a person the right to say bad things about someone else. Oh, he has a drinking problem, don't mind him. I don't think that's right. My only solution is to avoid the subject, avoid get togethers when he's around, sober or drunk.