I'm with you there. It places a conventional set of obligations on people that no other holiday does.
Give me the Easter break in preference any day: so much more relaxed, no set-in-stone collection of 'traditions' to be adhered to, can go away or stay at home as we please, and to top it off it' a beautiful (the best!) time of year with spring just bursting out and with nice, light nights.
That said, I'm an 'anything goes' type person who prefers not to be stymied by routine and everything being the same each year. My DH is just the opposite!; the one good thing being that spending Christmas anywhere other than his own home is unthinkable for him. (But I want to go to Hawaii!)
OP, your DS has autonomy of choice: he can come to you if he so wishes. Your DiL is fine with that, so from that perspective isn't trying to dictate what they both do. She also has family she doesn't see at any other time, which does make her position understandable.
You have a right to be disappointed, and I understand that you are. You would be very unwise to turn this into a battleground, though, and personally I'd bite my tongue and refrain from saying anything at all. Unfair it might seem, but when there's a clash of wits between a DiL and a MiL, the MiL rarely wins.
Could you perhaps invite them over to spend Christmas Eve or Boxing Day with you instead?