Summed up: be the bigger troll. There is a high likelihood that someone at your Thanksgiving dinner will be an asshole. (Hi!) Worse yet, there is a high likelihood that someone will be a particular variety of asshole that sullies humanity with some combination of stupidity, racism, misogyny, or otherwise noxious views that make you want to say or do something you shouldn't.
As I said last year, arguing is pointless. See Nyhan, Brendan (way more publications than anyone at that stage of his career should have-- dude's a badass). People with incorrect beliefs just dig in when confronted with facts. And vile moral beliefs? Don't bother. To borrow a great phrase from Neal Stephenson's The Rise and Fall of DODO, for these people, new ideas have "the weight of mist." So, don't waste your breath.
Provoke them instead. Mess with them. You have a relative who thinks that Roy Moore is being set up, and all those women are liars? That's right! Lying jezebels shouldn't ever be allowed to testify in a court 'cuz they're all liars! And hey! Age of consent?! Let's really bring back that bible! Polygamy and child marriage! YEEEEEHAWWW! Isaac and Rebecca, anyone? And hey, Jacob had to marry Leah and Rachel because Laban tricked him, and who the hell even cared about consent?! What did informed consent have to do with marriage anyway?! Those jezebels are a bunch of liars, trying to stop Roy Moore from bringing back the bible, and all hail the bible! YEEEEHAWWW!
Oh, you can have so much fun if you stop trying to argue with these idiots, and just troll them. Move past that Mary and Joseph crap and get to the creepier stuff. Now, it does require actually knowing what's in the bible, but that's the beauty of it. These idiots mostly don't! That's what makes trolling them so fun and easy! So, do a little studying, and go to town tonight! Whatever jackass relatives you have, figure out what their buttons are, and instead of arguing with them, press those buttons! Be the bigger troll!
And, as always, the Drive-By Truckers...