If they have kids they should be a little more understanding !
If I may be blunt, and direct, childbirth can be a sprint or an endurance event, either way your body will be bent like a pretzel, I'm not saying this to freak you out, just to say that unless you are blessed by the godess, less than two weeks later you will still be feeling the effects. It's not a time to spend all day in someone else's house, trying to smile wearing a nice dress and heels (because it's expected!) worrying about the baby, whether the baby's going to throw up on great aunt Gertrude, or catch a bug or whether your boobs are going to leak through the super thick breast pads that make you feel like dolly parton.
This is the time you want to wear comfys, and eat junk food just because you want to, you made a whole other person, you DESERVE pizza and ice cream !
Seriously though, is there anyway to work out a compromise ? This will sound a little bad, but what about saying to his parents that for baby's first Christmas, you would like it to be special, just you and your hubby, and the grandparents, lay it on thick, how much it would mean to you both to share the special moments just you guys. The memories that don't have to be shared with everyone.
Good luck with everything, finals, baby and in-laws.