Oh, Honey I feel for you!
My husband and I have been together 22 years and have known each other 27 years. My in-laws are always (and still do) playing the power struggle game with me over their only son. If I would of known how they were, I probley would not have married my husband, and we have a Wonderful, fun, loving marriage but his parents are a constant pain in my... side!
Unfortunately our teenage children don't have a lot to do with them either.
The sun sets and rises, according to his parents, on his bald little head...
My husband does stick up for all of us but his parents are just set in their ways...The other family members love us dearly and wonder how I have put up with them?!
See if you can find a bonded pet sitter that can either stay in your home or just come by several times a day to check on Monster. My mom has one she has come to her house to take care of her 3 bengal cats and she loves the lady who does it, and her cats are happier(they are all rescues)
If not for this time, find one for another time, or find a pet friendly hotel close by his parents and stay there?
Good luck and happy holidays,