Shelflife
I completely understand how you feel.
However I urge you to tread with care. So many GP. are at odds with their AC - please dont let that happen . You sound to be a very sensible woman and a wonderful GM . I agree it was mean , you should have been invited to the maternal GP home. However, don't let what happened damage your relationship with your son and DIL.
Good advice.
Tho it wasn’t at all fair to be left out there are two ways of looking at it.
Am presuming the other grandparents weren’t at the first birthday party, so it was nice for them to have a celebration later on.
Or… as usual the DIL and her family take precedence in these things and it will happen a lot in the future.If so, there’s nothing to be done, it is what it is, sadly.
And if you’re left out in the future stop helping clean up.