I agree with the title of this thread fishing4life, estrangement does suck but I'm afraid that's all I agree with.
We have been estranged from our youngest son for almost 10 years which is his choice but your estrangement is clearly yours and your wife's, and appears to be because you don't, and have never approved of his wife.
The hardest thing about estrangement from one's own child is two fold. It is accepting that in all probability you will never see your adult child again, and that you will never know their children, your GC.
You and your wife, due to your mature and compassionate d.i.l. have had the opportunity to have a relationship with your GC and you have thrown that away.
It may not be too late to contact her and say how sorry you both are, for not appreciating and valuing the fact that despite you having no contact with your son, she has gone above and beyond to facilitate a relationship with your GC. She may be prepared to resume her efforts but of course she may not want to put herself and her children through this again.
15 years is a long time and I do think that the longer the estrangement lasts, the less likely the possibility for a reconciliation but that is usually because there is no contact, and you have had contact with your d.i.l.
My advice FWIW is as I have suggested to contact your d.i.l. and write to your son. There may still be a chance if you offer an unreserved apology for the way you have behaved toward him, his wife and their children.
If you've ever read the other threads on GN on estrangement, you will have seen that there are many parents who have no contact with their AC and GC due to the influence of their AC's partner.
I have never come across a case before where it is the 'hated' d.i.l./s.i.l. who has attempted to keep some level of contact and ultimately been rejected because if her p's.i.l. can't have their AC, they'd rather have nothing.