So, trying to be a good sport I said "how about 4:30?" It would mean we would have 30 mins to eat, but our kids are small and not likely to sit for longer than that. No dice, in-laws are eating at 5pm and it has to be 4pm.
You probably realize this, but you shouldn't have even offered 4:30. That was your one day for in-law-free time, and they wanted to descend on you in the middle of dinner? Half an hour is not a lot of time to enjoy a special holiday meal.
"I'm afraid that won't be possible. We're eating at 4."
"Well they have to come at 4 as they are eating at 5."
"Well then I guess the timing is not going to work. We'll see them tomorrow."
This kind of bullying strikes a nerve with me because it's exactly how my PA BIL gets his way too. I've posted about him before. Everything has to revolve around HIS schedule, because, "It has to be at 4, because I have X at 5" and then if anyone says 4 doesn't work, he finds a way to make the alternative very undesirable for everyone else involved, so that everyone else agrees to his demands at great inconvenience to them, to avoid the Undesirable Alternative. I always want to say to him, "OK, go do your thing at 5, too bad we won't see you at our event" but the problem is I'll alienate my sister.
But I digress; I agree with the PP's that you probably can't work out your relationship
with your in-laws (they sound bat poo crazy) but your husband needs to understand that unless he's willing to let go of those apron strings and take a stand for you, it's never going to work.