Hi Irish. I can see positive and negative on both side of the question - Do I tell them? IMO, I am leaning towards letting your daughters know.
The behaviour of your XW and the things she said are similar to what my W has done.
My W justified all the damage and heartache she caused “because he’s (OM) worth it”. That had to leave a mark on the kids. If W moved on from OM, to another OM or on her own, it would greatly lessen the validity of her “reasons” for breaking up the family and leaving her kids.
The fact that your XW has moved on from her soulmate illustrates her confusion and eliminates her “reasons” for breaking up her family. Yes, XW moved on to OM2, however her original fantasy collapsed like sand. I think knowing that would be beneficial to your daughters.
Maybe the question is not - Do I tell them? It’s - When do I tell them?
I am glad that you found out now and not at the spring bbq from a chance meeting.
You have done a wonderful job and I have no doubt that whatever you decide will be the right decision for your daughters.
Feelings are fleeting. Be better, not bitter. Love the person, forgive the sin.