I was pretty proud of this so I thought I'd share, in case you're still trying to find something for your moms. My siblings and I are going to get her this with all of her grandkid's birthstones on it:
Birthstone Family Bond Charm Necklace in Sterling Silver only $35.00 - 50 Gifts Under $500
She's going to LOVE it, it's perfect!

And completely affordable, for the necklace and 4 bs's it comes out to be only $22/sibling - a steal!