Pony,
Your reply was the best one, nothing.
However, take a look at womensinfidelity.com, then pass it on to your MIL. It opened my eyes. Maybe your stbx would benefit from reading it, too.

Trust me.. no answer is the best... MIL are not known for their diplomacy.... let it be...

Avoid stbx MIL's invitation to the holiday gathering next year with your significant other. Start your own traditions and live your life away from their family except when benefitting the children. There is no reason for you to continue this relationship. sure you're only divorcing their daughter, but was at her request and you're just trying to put your life back together. I agree with the others, no answer was better than an "OK, or thanks for the invite, etc."
Moving on to greener pastures is best.

Who knows what your MIL was thinking.
There is no good response to a weird-ass comment like that.

I'm glad all was cordial, given your children, you'll have these people in your life.
But
I agree. Thanks but no thanks to the next holiday gathering.

Stay strong. Isn't is interesting how many "I told you so..." you get afterwards.

Keep a smile on your face and let your personality be your strength. cheers

i agree. i think MIL is trying to pretend that everything is just fine. i see from your profile that you have children together and that, of course, is a good reason to spend holidays together if possible. my MIL was just off the wall all the time. i was so happy that last time i saw her and realized that i didn't have to ignore her nasty, insensitive comments ever again! it would have been ill mannered, but you could always mention that adultery made you uncomfortable and see if that encourages her to keep her opinions to herself where they belong now. best.

My MIL told me I feel bad what you are going through but don't want to take sides. How do you take sides??? Her son is moving out and having an affair. Oh well, I didn't say anything.

I am shocked by that statement....sounds like my inlaws...and my parents.

Do these pple not have anything better to do?..Be supportive rather than wishing ill-will towards family members.

You are a class act Pony, what the hell could you say?

"We use to lay awake at night wondering how it is you to stayed together so long MIL?"

(btw..I speak with anger about this issue, I had 4 MILs during my course of my marriage)

My lawyer gave me the best advice..it doesn't matter how the in-laws feel about you, blood is always thicker than water. It doesn't matter how horrible the behavior it will be forgiven.
Nothing, was the best reply.

I don't think that any of STBX's parents or step parents blame me for what happened. I honestly think that my MIL think I would be the one to leave or end the marriage was because they thought I would get fed up with STBX not finding her way in her job and wanting to adopt all the children and all of that. I don't think they ever thought I was a bad person, because I never gave them a reason to think that. STBX's father even told me after all this went down that he could not have asked for a better son in law.

I also think that my MIL thinks that it would be ok to invite me and my significant other to their holiday gatherings, because her and her ex are still friends and attend each other's functions. It is kind of strange, but I think she feels if they are that way we can be too. It's too soon to tell. And I wouldn't want my SO to feel uncomfortable.

I guess they knew her better than you did. Sounds to me like they are nice people.

Probably she pretty disappointed in how her daughter turned out. And probably asking herself what role she had in that.