It's also extremely tacky to put ANYTHING about gifts in the invitation. If you read the etiquette guides, they say not even to list where the person is registered - that should be conveyed when an invitee calls/emails to RSVP and specifically asks about registration or gift ideas.
As far as getting invitations for relative's children that you don't even know - is it possible that in the spouse's family, it' considered polite to invite all the family? I know that several of my DH's aunts were offended that they weren't originally invited to my baby shower, because in their family, absolutely everyone was invited to everything. I, on the other hand, thought an invitation would have been seen as a rude request for a gift, since I'd barely ever spoken to them. ;)