Hi claudia! Well, based on what you wrote here, it does not seem to me like you are in the grip of FOG. You sound totally reasonable and aware of the issue and what you want.

It sounds like you know what you want to do and I know I would be annoyed with your mother's assumption that you will be able to spend time with her for the Happy Family Photo Opportunity she seems to want. There seems to be an underlying assumption on her part that you will be able to carve out some time to spend with her. that would annoy me to no end.
Here's what's kicking me, though. If we didn't have other plans, I would not begrudge my mom and dad some significant time spent with them. I've got pretty good boundaries in place and can actually enjoy them in limited doses. I would be okay with working out something so we can see them, ordinarily. It's just a really poorly-timed visit and I'm not sure what my actual desires are and what is FOG trying to push me around.
Well, it is an awkward situation regardless of who is involved and the history of the relationship. As you said, you would spend time with them but the fact is, your mother's request is too late. I see nothing wrong with your line of thought.
(BTW... .a very Merry Christmas to you and yours!)
Maybe you can suggest a get together after Christmas?