Is there any chance that Darling and his wife would be willing to make a stand? I realize it's a lot to ask and may sound needy, but isn't that the kind of thing you do for someone you're planning to marry?
It's possible that him telling his parents that he's choosing to spend Christmas with "his family" (referring to you and the kids included) will snap some sense into them. Some people are used to getting their way, and can be big babies when they have to accommodate the needs of others. They're grown-ups and they'll probably get over it.
But either way, you won't be "alone" on Christmas. You'll have your kids, and your kids will have you. That's a lot more than some people have...
Also, "Christmas" is just one day but "The Holidays" last for quite some time. Welcome to Modern Living. Dates are irrelevant. Just make sure to plan some special time for your immediate family, and focus on the positive.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker