An invitation, whether you know the person or not, does not mean you have to go or buy a gift. What I think they did was just use the church rolls to make out the guest list. I have seen people cast these wide nets (my sister went to a co workers daughter's shower to be nice because she figured if she, not knowing the girl was invited, they were hard pressed to find guests. HA! 150 people in a huge hall, not enough food, only a select few got favors, and she didn't open the gifts because there were too many) and they are to me, blatant gift grabs. And then for her to register for a full nursery for her house? No, unless she was raising this child by herself. In my opinion, you don't even have to send a card, as this is not even the mother of the baby they are celebrating, but the grandmother. Send regrets, and don't give it another thought. As for your mom, she sounds like my former MIL...didn't matter who she got invitations from, she always HAD to send a gift because what would people think? Half the time she had to call other relatives to find out who this person was.