What is the child's relationship to the father like?

I'm sorry to hear of this situation. Somehow your step daughter-in-law became toxic in her behaviour over time.

Is she an accomplished person in her own endeavours herself?

It would be nice at least that there would still be family gatherings.

Some distance for awhile is necessary.
There's not much you can do re your hubby-his son relationship. Somehow I get this powerful feeling that even if son wanted to improve closer relationship, he is afraid of more disruption/misunderstanding from wife at this time.

Sounds complicated.
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