I mean you could just get yourself uninvited by divorcing the cheating husband

Sounds like his family are trying to tell you something. Follow through and divorce your cheating complicit husband, and be glad to be rid of all of them.

Why on earth did you stay with a cheater in the first place? You realise she's probably still boffing your husband as well as his brother now, don't you?

So from now on go only to your family’s holiday festivities (with your husband). Problem solved…. While it’s odd that she’s now a sister in law, there is nothing you or your husband can do about that…. I am assuming you and husband already sorted out the rest of the fallout from his prior affair and this thing with you in laws by itself is no reason to divorce him.

Two questions come to mind: Why are you still married to him after finding out he cheated on you and how did your brother in law come to the conclusion that it was a good idea to marry his brother's mistress?

And you’re still married to this dipshit because…?

She's a ***** and your cheating husband is what? Forgiven? Either forgive them both or divorce his cheating ass.