In my view, if you're having a family get-together it's better to just be nice to your denier relatives. Anyone who still rejects climate science is a lost cause on that subject.
Family get-togethers can be trying enough without adding more tension. In any case, if you don't know yet what values you share with your weirder relatives then you probably never will.
Instead of explaining how devious disinformers have played a hoax on them, find out what other conspiracy theories they entertain. Shift the conversation to something more innocuous that will amuse you and won't get you worked up, like how they know that gravity is a hoax :)
If you have the opportunity to take some time off from work (forced or otherwise), make the best of it.
I'll see you all back here in 2019.