As usual I agree with @Aquamarine1029.
In my experience NC is the result of a toxic relationship and the decision was not easily arrived at by the people I know.
You sound like a very kind woman and that makes you vulnerable.
I am not kind and if these people have as you write, been unkind to your children, they could die screaming as far as I am concerned.
My advice to you is to absolutely NOT get in any way involved.
Nod sympathetically if you must at your husband, but offer neither advice not solutions.
Personally I would be very very careful of allowing him to burden you with details of his parents.
He has made his choice to remain in contact with people who hurt his wife and children so his judgement IS warped IMO.
His parents growing old, as we all will, changes nothing in my view.
It is in yours and your childrens best interests to remain completely uninvolved.
Any weakness on your part could see your husband asking can she visit.
His involvement with her should remain completely separate from you, your children and the family home.
His parents chose to behave as they did.
Your husband has made his choice.
You wisely chose to finally protect yourself and your children from their choice to behave in a toxic manner towards you all.
Do not go back on what was a hard painful decision.
Some years ago my friend did and bitterly regretted the damage it did to her marriage as it dragged up huge bad feeling from years ago when his awful mother started coming for lunch again on sundays.
Her husband had put pressure on her to be kind to an old woman in her final years🙄.
Long story short my friend started to absence herself from the home, (her children were away at Uni), on Sundays and told her husband that she was actually very angry with him for the pressure on her to go against what she wanted, and she wanted to separate.
That woke him up very quickly, but there was real damage done to their relationship and trust, for a long time afterwards, and it changed how she felt about him to this day.
Tread with real care and continue to protect yourself and your children first.
Men often suit themselves and believe me it is highly likely he would like YOU to share any caring duties that may arise.
Any ambiguity in your position regarding his parents, leaves you vulnerable to being used.