Help me with a plausible lie to get out of a holiday get-together

Mmm… I can’t speak for the OP, of course, but family interpersonal relationships are usually embedded in a twisted nexus of cognitive-dissonance contradictions.

It’s quite possible that they have picked up on that obvious fact, but refuse to accept it-- or perhaps it’s more accurate to say refuse to accommodate it.

If they have the popular traditional “family values” mindset, they may well see an alienated or hostile relation as a “Scrooge” who is crying out for intervention, and regard attempts to avoid family gatherings as an invitation to step up intrusive persuasion.

Put another way, they may harbor a generally-approved “We Are Family!” social fantasy in which certain gung-ho family members are “enforcers”: would-be Good Shepherds who will seek out and reel in stray black sheep with a vengeance.

They insist upon the reluctant relative participating on “principle”, willy-nilly, in order to fufill their abiding group fantasy (or “ideal”) of family life-- i.e., no “out-grouping” allowed. Crazy, ain’t it?

Glad you survived, OP. FWIW, I have a cousin I can’t stand, and it’s taken over a decade but I think my mother has finally accepted it. I mostly don’t go to the family gatherings this cousin attends; she doesn’t attend many of them, and on the rare occasion our paths cross (like when her brother, whom I do like and didn’t want to snub, got married recently), I just avoid her. A combination of honesty, in advance and in private with the people I care about (like my mother), occasional small concessions at my discretion, not in response to pressure (like going to that wedding), and standing firm on my choices not to go to things I don’t want to go to, without being drawn into an explanation/negotiation, have finally gotten me where I want to be. Best of luck in continuing to navigate this family dynamic.