I can see why you’re frustrated. I come from a big family myself, with our share of “colorful” relatives. The general rule was to let others deal with their own little dramas and let the chips fall where they may. It’s disappointing when the relatives can’t get along, but sometimes direct control is not a viable option. Sometimes the most peaceful alternative is to just ignore it.
This year, your husband (and to a lesser extent, you, because in-laws are wise to concede their standing in these things to the spouse more immediate to the drama) are the hosts, the on-site referees. Obviously, you wouldn’t politely ignore the relatvies if they started throwing punches and furniture. You do have boundaries of what sort of behavior you will tolerate, then. The only question is where that boundary needs to be, and how it is to be enforced. You decide who is invited, for what, and when. One of my relatives chided an older guest who presumed to smoke in his new house during the housewarming. I didn’t hear the manner in which he did it, but they were not on speaking terms for a year or two after that. I don’t think either one cared.
Generally speaking, though, most extended families decide to endure each other without any interventions by a referee, as long as no one is drunk, under the influence of drugs, breaking things, or physically assaulting the other guests. I’d let your husband be the head umpire, deciding what the boundaries are, how to enforce them, and whether or not he wants an alliance with other siblings before handing down warnings, penalties or ejections. He knows the game better than you do.
As for the Facebook thing, just stay out of it, except perhaps to remove the offender from your “friends” list, before he gets around to attacking you. How others manage the distribution of their own news and gossip is their issue, not yours.