Quick question did she forget the other grandkids b-days?? Or just your daughters? If just your daughter you have every right to be mad, if she forgot all of them, then she is just a forgetful person. (or cheap)
I feel your pain, my husbands family is infuriating and your husbands mom sounds like she could be besties with my M-I-L! There's 18 grandkids (3 of which is our children) well a couple of years ago, on a visit to the MIL's house (everyone seemed to be there) we found out the M-I-L knitted these really gorgeous blankets for the grandkids, EVERY single grandchild EXcept for our kids had received a blanket! This is just one example out of many. I gotta say tho, my F-I-L seemed to be a bit embaressed by the situation and told my kids to tell grandma the colors they wanted for their blankets. My youngest has gotten hers, I doubt if my 2 oldest will get theirs, the M-I-L's health is going down fast and plus our oldest 2 happen to be "step" children. (to my husband they aren't step children, they are his own, he's been with them since they were still in diapers. For some reason his mom doesn't see it like that.

Even tho her current husband is 7 of her kids step dad, lol that's what has me confused. Her kids were never treated like step kids, they were treated like his bio kids)
In laws can be so infuriating, and what's worse is when the M-I-L is "queen bee" and everyone does what she says and just lets her run the show. No matter what or who she upsets.