This is an interesting dynamic that I have a good deal of experience with . . . not just with my own family but that of clients that I have dealt with.
If a Mom has not got a good dynamic with her own mother in the first place . . . when that Mom has a baby a whole load of issues seem to come into play. The Mom is still wishing/praying for all these things that she never got from her own mother . . . she perhaps privately hoped that when she had her baby her mother might swing into things and finally be the mother she wanted her to be. When this does not happen . . . it does not matter who else is sweet and kind and thoughtful to her . . . she resents it deeply . . . she wants it from her own mother. Someone else being kind and sweet and supportive instead of her own mother, in her eyes . . . just seems to make everything so much worse. The resentment to the kind sweet supportive person is very real and tangible.
Once you can try to see this Nanahana it might help you to observe things from a slightly different perspective.
It can and does change over time as it has done in my case.
I too at points felt like throwing in the towel, but eventually just kept up being sweet and kind and supportive when I could . . . but a little bit staying in the background.
Eventually the Mom realises and eventually over time appreciates what you do for her little family and things get better . . . as to be honest she is not getting that kind of love and support anywhere else/and may never achieve that i.e. not from her own family.
So hang in there for your son and your grandchild and for Mom who is going through it all at the moment.
If you were good before . . . that hasn't gone away for good . . . it could well return. We can never underestimate how terribly useful us grandparents are to our sons and daughters and our grandchildren
Our Welfare State. Is it broken?