You need to take the baby steps with your h to get him away from these awful people
Firstly you make sure that neither you nor your kids ever see them again, and you should get the Susan Forward book(s) Toxic Parents/Toxic In-laws and read together
It’s not his fault that they are like this, they will never change.
What I find particularly chilling is that she did this to you in front of an audience and not one person batted an eyelid, it’s like she has goaded you beyond the beyond and wanted to see what anyone would do to protect you, and nobody did a thing
If she can do this to you now, what next?
Let me tell you what next... it’ll be your kids. People like her hurt others through their loved ones, she’s got away with it with you, she’ll need to up the ante, your kids are next in line.
Your h will need help to get the fog to lift, but you have to protect your family now
These people won’t ever be coming to your home again, and you’ll be “busy” every Christmas, birthday, holiday, Easter and Hallmark occasion
You’re done, your kids won’t go there without you and hopefully your dh won’t go there again either
Perhaps suggest to him that he makes a promise to himself not to initiate contact or visit them for x number of months and to then evaluate how much better he feels without them in his life
Absolutely yes to therapy, he won’t be able to heal without it.