I would appreciate a MIL support group too! I picked up a tad of resentment since the day DIL met my son. There is no competition in my eyes, we just both love my eldest son and I was so happy he had found the right one for him.
Over the years she has managed to squeeze me out of the family and now has my son to herself who doesn't like the situation, says he knows why she is like she is and wants a quiet life. I've been supportive to her, not interfered but helped all I can with the GS's...only when called for.
However she has a bad relationship with her mother, her father lives a long way away with his new wife, she falls out with her sister regularly, always takes the mick out of my other son [who has done nothing] and is jealous of my other DIL who is great [has no issues].
So I think it's in her personality, she feels the world is against her. It isn't, but she pushes people away. And has been very hurtful to me. I'm waiting for an apology which will not happen, as she has made clear.
I would agree with other posters that in this situation to keep quiet, love your GD as much as you can, keep texts for future reference and not bite when provoked. I hope it works out. GD has a mind of her own and may very well end up living with you OP when she is older. I've heard of many situations where this is the outcome.