N., first let me just validate your feelings! The same thing happens in my family with my niece. I know how frustrating it is to be told "You just have a chip on your shoulder", or "I see your kids all the time", or here's my favorite.."jealousy is an ugly thing". These actions are hurtful and I am very much feeling your pain of wanting to share milestones with your mother but only feeling angry after doing so. What I have started doing is having my children call Grandma themselves. They can feel proud by sharing their accomplishments and most of the time I'm confident that Mom wont spoil their excitement by talking about their cousin to them.(although beware, it has happened a time or two, but not nearly as much as when it's me)
Confronting Grandma, our case, would not do any good. She tends to be very defensive and things seem to get really dramatic really fast. In order to not play the drama game, this has become our m.o.
The dynamic in my family sounds very similar to yours. We are a successful family who has never asked for much of anything from our parents, but EVERYTHING my sister's children do is a MUCH bigger deal than anything my children do. If my mom has plans with me or my children, she will drop everything at a moment's notice to be with my sister and niece.
I hope that you can get to a point where it doesn't matter what anyone else in the whole world thinks of your children as long as you are their biggest fan. Even if that means overcompensating with affection after Grandma has dropped the ball. Trust me, I know it's not easy. I'm still working on it myself, and it's more than a little disappointing to have to lower your expectations of your own mother, but I'm much happier having done so. If you're not happy, your kids won't be happy and maybe that means distancing yourself a little...after all "absence makes the heart grow fonder". I don't think there's any easy answer and I hope I didn't ramble too much, but I'm here to validate your feelings and let you know that it's not ok for Grandma to behave this way. I don't think they realize how painful it can be.
Hang in there-- and with a parent who is so in tune to family dynamics and their children's feelings I'm sure they'll turn out to be great kids with or without even handedness from Grandma.