Nansnet
It's a family tradition where everyone is invited. As you said, you can't simply un-invite your nephew and his family just because your sister has had a falling out with them.
I'd tell her that, as always, the invite is open to all, and you wouldn't want anyone to be left out. However, you need to tell her that the issue between her and her son is nothing to do with you, and they need to sort it out between themselves, and decide what they're going to do about it. It's not right that you should be put into a position of having to choose between certain family members. Let them sort it out and decide whether or not they can act like adults, and not create a bad atmosphere, and spoil the day for everyone else. Or, one of them chooses not to come. Their decision, not yours.
And I'd also be telling them that if one or the other causes any upset on the day, then you'll have to ask them both to leave, as it's not fair on everyone else.
Hope it can all be sorted out before Christmas!
Thank you! I will likely end up sending a message to her shortly before I need to finalize a head count. I am drained. A long term health battle will do that. She knows I hate drama. I won’t chase her to come, though it appears she expects this of me now as well as her son. I love her but I am too weary to engage her here. For once I just wish she’d think about the impact her attitude has on our entire family