N.A.
This is N.'s husband speaking. She wanted my opinion. Here goes:
What is the problem here? Is it the toxic family? The traveling? There are many issues here in this letter. From a guy's stand-point: pick a topic.
My two cents is this has NOTHING to do with Thanksgiving. This has everything to do with your husband not defending the castle, speaking from experience with people hostile to my wife. I read your other post as well and it sounds like the issue you outline in the last paragraph has been going on for a number of years with his mother. I learned the harsh lesson of saying ANYTHING bad about my relationship to my mother. I saw it N.'s eyes. I saw the hurt. It is his JOB to be the go-between. His first duty is to you and the kids, not to his mother and family anymore. Sounds tough, but oh well. It comes with the job.
My advice to him: Don't go. Stop speaking to Mom for a while and re-connect with Dad. Chances are if Mom is putting sisters first, his Dad is getting run over.
As far as the letter goes, you know you husband. I wouldn't react too well to reading this stuff in letter format, but that is not the relationship that N. (my Abigail Adams) have. And don't invite them either. At this point, it would be like throwing the gates open to the Hun, letting them trash the interior, then they leave you and husband to sort out the pieces. Work on his defense of the walls first.