OP,
I am in my late 50's and from reading your situation it reminds me of something similar that happened my friend years ago.
Her husband's brother behaved badly, had an affair and his marriage broke up.
My friend was appalled at her in laws and how they moved forward, supporting their son and his new partner, whilst making no effort whatsoever with their ex daughter in law and their grandchildren.
My friend said it was a huge wake up call for her.
She actively supported her sister in law, whom she wasn't very close to up to that point, and she pulled back MASSIVELY from her inlaws.
She told her husband that the ease with which they had abandoned their previously lovely DIL was not something she would EVER forget.
She told me that she hadn't a notion of running around after them going forward.
She still remained polite and respectful and kind, but she really no longer intended to invest in the relationship.
Her ex sister in law remarried within a couple of years and moved a small distance away with her children who didn't see their grandparent's more than once or twice a year and 15 years later they are not close.
Her sons new wife never had children.
My friend has left things completely in her husbands hands since and never got involved in their elder care.
You have shown your daughter who you are.
That is, a woman who would support a sons poor behaviour ahead of the woman he married.
She does not like what you have done.
She no longer respects you.
You have shown her how you might behave if it happened to her.
Like my friend she has decided to pull away and no longer invest in the relationship.
In your place, a good woman would reach out to her ex DIL and explain that whilst you love your son, you do NOT support his behaviour.
That you want to support her in any way you can.
If you don't do this, and your future DIL was MY daughter, I would be very unimpressed with you as her prospective in laws.
I would be very disappointed at my child marrying into such a family and I would advise caution.
It sounds like your DIL is a bright woman.
YOU have shown her who YOU are, by your behaviour, and she doesn't like what she sees.
You need to focus on YOUR behaviour and your unswerving support for your precious son, and how that looks to others, if he has infact behaved very poorly.
Stop focusing on her and start looking at yourself and what she sees.
If you don't, YOU will be the loser in this situation.
Goid luck.