When your son married his wife, she didn’t agree to ‘join’ your family and he did not agree to ‘join’ hers; they agreed to create a new, nuclear family together. Unfortunately this is proving difficult for you as your son has now ‘flown the nest’
They do say a son is a son til he takes him a wife, a daughter is a daughter for all of her life.
When sons grow up, they will often distance themselves from their parents and take their place as head of their own family.
Daughters radiate towards their own mothers, especially after having a child, as your mother is normally your most trusted person in the world and you have a lifetime of love, nurturing and experiences with her so it makes sense that a daughter will go to her when navigating motherhood herself.
Unfortunately you cannot dictate how their nuclear family chooses to spend any holidays, nor have any say in who they spend their holidays with.
The most you can do is extend an invitation to them OR say that if they are available for any holidays then you would go and visit (but don’t be visibly annoyed or dismissive if they say no) but they have their own family now and you need to just respect that.