I feel for you. I meet son's MIL from time to time when we bump into her at their house. I don't think she really likes me. My DIL is lovely and we get on well. She is very close to her mum, and I try to think that her mum must be ok to have brought up a lovely young woman.
Other MIL was the at birth of both GD's and we didn't see them til much later. I think this was right though, I believe in giving them time together as a new family.
Other MIL sees the GDs much more often than we do as they live nearer, and the children stay overnight, but have never done here. They are on their 5th annual holiday with DIL's whole family, and I am wildly jealous, though I understand how close DIL is with them. They only get so many days off a year, and I can't envisage them ever coming with us.
The elder one asked me why I never go with them. What to say? I said it was a holiday for them with mummy's family, and she said "But you're our family". How sweet, she's only 5.
However we do see them almost every week, and we have a great time. They never want to go home, and the little one mouthed through the car window last time, "I love you Nanny".
So you'll just have to see how it goes, and manage your feelings, while letting them know how much you love them all and dearly like to see them as much as they can manage. The baby is still relatively very young, so don't fall out with them, they're probably still too overwhelmed to think of dishing out equal time at the present.
Like I say though I can't help feeling jealous, and I had a feeling that this would happen from the start. There's nothing I can do about it. Many of my friends are in similar situations, or worse. I have posted before about being a paternal grandmother, always harder I think. DH keeps quiet on the subject!
Actually, I'm glad to get this off my chest as well.
I hope things get easier for you as time goes on. 