M.L.
Personally, when I was growing up, all I ever got from my family members was birthday and Christmas cards. And usually, only the birthday card was specifically addressed to me. The Christmas cards would come to the family.
I can't believe that you send cards for every holiday. That seems a little excessive to me, and it's completely rude for his sibling to complain that it's not enough for you to send just one. They're lucky you care enough to send a card at all!
If it were me, I'd probably just send one card to the entire family, with the kids names on it and leave it at that. The kids will be happy to have been thought of, no matter what, and the parents are the ones with a problem.