I definitely got cards with cakes on them… from my grandpa, friends, probably my parents, other people, … all of them knew I had diabetes. I never took it “personally”, actually I’ve never thought about it until I saw this discussion.
To me, a cake on a birthday card just means “happy birthday”. Neither more nor less.
And, to be honest: we are allowed to eat cake, aren’t we?
We don’t know your sister, so we can’t tell if it was supposed to mean anything apart from a simple, nice “happy birthday”. But I would never assume that a person sent me a card with a cake on it to offend me.
We are all different but I would be more offended by a card that clearly addresses my condition - not one that clearly does not.