I'm feeling rather sorry for the dog at this point.
The trouble isn't the dog. The dog doesn't know better, evidently. The trouble is the people. (We raised our family of children, and now we're raising dogs for a service organization, and our grown children say we're favoring the dogs over them! But we teach our dogs good manners.)
If the dog hadn't been interested in the dinner, something else would have happened. It sounds as if visiting your relatives is a real adventure.
Don't worry about little things like Facebook. Let your relatives do what they please with their FB pages. If they're writing about you right now, you don't want to see it anyway. You have better things to do. Don't waste your time stewing about them and being bitter. Bitterness will spill over into your own home. Can you find any humor in it to laugh about? (I can, reading your narrative.)
And please seriously think about investigating Al-Anon to learn how to deal with your alcoholic MIL. I think that's where the real problem lies.