The fact that she has no contact with her own family is a worry, but perhaps they are dreadful. In either event it does increase the risk for her cutting you off too.
However as to you not being invited, I never invited my mum (or MIL) to my daughter's parties, they were child focussed and quite the effort and kerfuffle. My grandma (mum's mum) never came to my parties, nor on my birthday that I recall. She always came for Christmas though. We saw each other at other times and she sent presents and a card. We all got on really well. I don't think that was unusual and I don't recall grandparents at my daughters' little friends parties.
I've never thought about it but if I had, I might have decided against it because I wanted DD to interact with her little guests rather than Grandma, Auntie Boo etc. Also as you are her MIL, not her mum, she might feel awkward about delegating jobs to you or ignoring you in the rumpus.
Be very careful about betraying your feelings on this to your DIL.
How were things when your DGS was a baby? The birth of a first child is often heralds a deterioration in relations sadly when different parties have different expectations and norms.
