...Well, those are quite the descriptors for your folks. I'd say the stepdad "ruined Christmas" by starting it, but I don't know the circumstances. Hopefully your mother didn't start anything as well.
The secret is to not engage in any debate. Bring an airhorn and a pack of Christmas crackers, and whenever someone starts saying something stupid start blasting and setting them off until they shut up. They can't fault you for being jolly.
"Guess I ruined Christmas. "
There is no reason to stay silent if provoked. "Christmas with the family and relatives" is the very definition of bigotry for me. Solemn, ceremonial with enforced "happiness"
Bad feelings guaranteed with smiling faces all around.
Not gonna lie: It is a relief having left this behind a decade ago.
Also hopefully, New Year's is better for you.