I guess I could understand if they'd all grown up together and gone thru school together.  I guess.

But, this would seriously bother me, too.

They snubbed her and they knew what they were doing. If it was innocent they would have told her that they had to leave early as they were meeting a friend (if they wanted to avoid bringing up the ex). Perhaps they were trying to be sensitive but they just ended up being even more rude than if they'd told her about it and not invited her to join them.

I think I would mentally move all of them onto the list of people who I can't trust and who are more loyal to his ex than me. I'm not sure how I'd handle the next holiday gathering, it would depend on how they treat me in the intervening time I think. 

That is extremely rude, so my first reaction would be to avoid these people in the future. If that would create a lot of drama, I might consider going along (partially) with the advice above and remind these people that the purpose of the holiday gathering is to spend time with relatives and they should stay a while while after dinner--can they arrange their time with the ex either for earlier or much later (or another day)? There would probably be pushback, though . . . and there you'd have your drama. 

I don't know why so many people in our culture think it's ok to crap all over second (or third) wives. 

When things have gotten too complicated here with the holidays, we literally head for the hills--rent a cabin and say bye bye!  

I feel like it's fine for the wives to continue to have a relationship that stands on it's own with their brother's EX.  

I think they snuck off because they didn't want to make it obvious and thought it might hurt the LW's feelings.. which it somewhat has now that she knows.

I think given the treatment that has not been unkind prior to this that she should pretty much take the advice (though if there are several siblings.. it may be years before it rolls around to her again).  Let the wives know that if they have other people they want to see that day.. that they can let you know so that you can make sure there isn't too much conflict.

As an aside.. since you know that they are friendly with the ex.. these women are not suitable for you to share secrets.. vent to.. about the ex or your husband.. or anything really.  They have some loyalty to the ex.. it should be remembered.  Of course... as long as everyone is being nice.. no reason to shun them.

These people are not your friends, and if family you don't need them.  I would never invite them again to anything I do.  Or my family does birthday , party, events.   Make new friends, some people who are loyal to you.  This is a pit fall of second marriage.  Family and friends choose sides.  You know what side you are on.  
'real people whole of told you they were goning to BM to eat . 

That would be the last time I hosted a family feast. I would have been very angry and felt very insulted if they just all upped and left after the meal I had prepared. They treated your table and your home as if it were a restaurant,  OP. That is such a slap in the face! You were good enough to feed them but not spend any time with.

If they wanted to spend time with BM, then it was on them to set it up some other time, not to insult your hospitality.

this article in WAPO and the majority of responses felt that, because OP didn't say she didn't have an affair that busted up the marriage, she obviously did. And if she did have an affair her poor treatment was completely deserved. 

I just don't buy that. I had nothing to do with my DH's divorce but because he was estranged from his children the assumption was that I did. I resent having to defend myself on that score.

And even if the husband did have an affair and then married his affair partner she doesn't deserve to be treated badly because of it. 

You missed the best part of the response, which was at the beginning.

"It was a spectacularly, thoughtlessly crappy thing for them to do to you.

I’m so sorry — both that all the wives weaseled on you en masse, when you were super-vulnerable as the post-divorce new spouse on the spot for a holiday feast, and that your husband didn’t immediately take the initiative to see that you were made whole once you figured out what had happened.

Anything short of an apology line snaking out your door was insufficient. Ideally in a downpour.

And … I’m not done yet … you can also think of it as a sad and, in a way, beautiful thing that was stupidly, needlessly botched."

Carolyn Hax definitely thought what they did was wrong. She is, however, a big believer in ex's getting along - so as she worked through her answer she did end up with those final words. (Her ex still illustrates her column.) Most of the responses I read were supportive of the OP, not the ex.

one of the members of a facebook group posted a link to this on Washington post. Of course, given the group, it was nearly 100% all about the fact that they believed with certainty that the OP was an affair partner.

Just because the "love" for the BM was so THERE. But we here know that the BM can be the worst person on the planet, and because she dropped a few DNA samples, there will be times that the family embraces her over the new wife.

That in second marriage there could be family problems.  The family could feel that BM was treated dirty by her ex and you.  That they don't want to end there relationship with BM.  
You can not control other people.  If they want to be friends with BM  then let them know you will not be part of this dysfunction.  They must choose. BM or you.  And you must adjust your life accordingly.  As no big family dinners. Let BM do it 

Take wife/ex wife out of the equation. Now it looks like the mean girls ditched the new girl. It's juvenile and rude.

If they want to see BM, fine. But pick another day.

Going forward, I would be polite but that's it. I'd really have to think hard about hosting again.