Have you talked to your DS about any of this? I don’t think so, you’ve said repeatedly that these are all your musings and thoughts.
Why don’t you tell him how you feel? Why not say that you mind never having all your children and their partners together on Christmas Day? Explain to him that you don’t think it’s fair that your wish to do this, even once, is held to ransom by the random single sibling? That you feel rejected by DIL, used for a second Christmas celebration such that she and her family get their way and you never do?
What would happen if you told him any of this?
Ultimately, there’s nothing further you can do once you’ve set out your position. You’d have laid your cards on the table and there would be no room to hide - for anyone - at that point. Any further rejection would be real.
I think your DS is taking advantage of you, your DIL is taking advantage of you, and you’re allowing yourself to be pushed hither and thither. It’s not acceptable that your other children and their partners should have their christmases moved around to accommodate this single sibling who they’ve probably not ever met, even.
(As I said upthread, I’m very familiar with the single sibling issue. It’s caused so many issues within the family let alone in the in-laws’ family. And, you can bet, if and when that sibling does ever get a partner, they won’t be seen for dust. Awful, spoilt behaviour. So immature)