This only comes up once a year as all the weddings were in the same week (different years). I've never sent anniversary cards to anyone except my parents.
Dh and I have now been married four years and have just been given an anniversary card from Dhs brother and SIL. Their anniversary was last week (they've been married longer than I've known dh). We have never given them a card, dh didn't before he met me, but they keep giving us a card every year and I think the implication is that we are a bit shit not to reciprocate.
Tbh it never crosses my mind until they had us ours, an then I forget about it.
I have two married siblings and we've never exchanged cards.
Dh has just asked if perhaps normal people DO give anniversary cards and we are a big fat F for fail.
I'm putting it to you lot for opinions. And if the consensus is that most people do in fact give cards every year then we will pull our fingers out and do it next year.