Hello and thank you for contacting Just Answer:
It sounds like you are hurt by Stacey not being around. For you, as the mother in law figure, it is a very delicate situation. There is not much for you to do except be supportive of your son. Any other kind of intervention may be seen as a blurring of boundaries or meddling.
Unfortunately, I think this is for your son to figure out and has little to do with you, even though you care deeply about the situation. He may want to explore some of his relationship issues in therapy as you mention that this has happened to him before.
It is unfair for the granddaughters to feel guilty. It seems like your son may need to handle some of these issues on his own and not drag you or his children into it.
I hope this is helpful and best wishes,