I grew up in this situation. I still have to not mention to my mom that I'm visiting my dad. It sucks donkey balls.
Just to make sure that I'm understanding this right..
They are adults, and are visiting their father for Christmas. He is the WH, and mostly absentee father. They will not be visiting you for Christmas. Does that sum it up?
That is definitely painful for you, but I don't think they should have to give up their father or feel bad for visiting him at Christmas. They still see you too, right, but not this Christmas? Being an adult kid of divorced parents who live in different cities means you don't get to see all the parents at each holiday. After awhile, trying to fit everyone in makes holidays not a holiday for the kid. My sister was smart--early in her marriage at age 26 she announced that Christmas was at her house with her children and anyone was welcome to come. Houston, Denver, Louisiana, Arkansas--it's impossible for us to see everyone on Xmas Eve/Xmas day like some families.
I think it's a tough situation all around, and a tough transitional age ANYWAY as children start having choices and new traditions.