The only thing I can really suggest is formal family counseling. Hard to get people into that room, but if you do, it is likely to help.
Short of that, get into counseling yourself and then brainstorm with the counselor about steps to take that might help.
Maybe just get into counseling with the young mom. And, you know, let the therapist guide you about how to approach the delicate issues about her mom...
Also, generally, you might want to place your expectations, not on a fully resolved, perfect and wonderful relationship...but on small steps and incremental progress.
You know, start out small, just exchanging cards on birthdays and holidays. Let the relationship slowly re-build. Now that is my two cents...but a professional, obviously, knows much better. I was just suggesting that the path forward may include some very simple steps that are easy to accomplish.
Best of luck to you.