Awww sometimes family can be so cruel and awful. You can bring them up in a loving environment and they grow up seemingly impervious to it. I'm guessing you're writing this post not having heard a peep from them over Christmas?
Might I suggest you try cutting out the middle man (and woman) and write directly to your granddaughters? If they're not old enough to be on social media and phone or email's a non-runner, why not write a good old fashioned letter to them?
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Dear Lord God, please as an omnipresent all powerful God, we ask that you intervene in this stupid, petulant situation and restore some love in this family. It would be wonderful if the relationships between all parties could be restored but if the son/daughter's hearts are too hard for that, I pray Lord that a line of communication can be opened with the granddaughters. It must be heartbreaking for poor LL, please Lord your intervention is needed.
Amen.