OP, your post could have been written by/about my brother. He cut off our mother after years of verbal abuse and toxicity. I was around 8-10 when it happened so never really understood. For a few years, I did pester my brother to make nice with Mom. When I got a little older, I tapered down the pestering but still invited him to gatherings with Mom.
When I was about 16, my brother sent me an email and laid it all on the line. He basically said "You were too young to remember, but X, Y, and Z happened. I am not angry at Mom anymore, but she has hurt me very deeply. I cannot be around her, because I do not trust her to not hurt me again. Please respect my feelings on this."
It worked. I never did it again.
As for family members, I don't have much advice. I know that my family members constantly pressured my brother to reunite with Mom. Perhaps you could start a tradition of inviting select family members to your home around the holidays? That way they don't have to worry about throwing two separate celebrations.
And if your family members nag you about your DH's parents, I would come up with a stock phrase to give them. Something like "I appreciate your concern, but this is a personal matter. Please don't bring it up again."
For the record, my brother and my mother reunited about two years ago, after 10+ years of not speaking. Brother initiated it of his own free will, and was (still is) exceedingly cautious. My brother keeps enough distance that if Mom goes off the deep end again, he can leave the situation and pull back from the relationship
. As for Mom...maybe she's changed. The dynamics are a long story. But at least she behaves herself these days.