Kate,
What a thorny predicament for you to be in! A lose-lose situation all the way around, isn't it! And Christmas is such a stressful time anyway, you don't need all this extra emotional blackmail going on as well.

There's so much wisdom in this group, and I'm sure others will respond with suggestions. I have one thought, although it might not be all that satisfactory to everyone involved.

You could enjoy a nice joyful Christmas morning with your two youngest children. Then what about going to a homeless shelter and SERVING Christmas dinner altogether to the homeless people. You can include your son (if the shelter will allow him in)...and I don't know if your Dad can be accomodated in his wheelchair, so your Mother probably won't be so keen on the idea, but maybe SHE can then have your oldest son over for Christmas dinner at her place while you're helping at the shelter.

I know lots of families that started doing that to avoid the sort of impossible situations and stress you're talking about, and loved it so much that they can't imagine spending Christmas any other way now.

It would be one way to avoid and alleviate the problem for at least this year.