I think I would send a card. I'd tell my husband about it. If he felt that strongly against it, I still think I would still want to send it, as this arguing doesn't really involve you. I'm wondering, though, how the SIL feels about your brother (and you) and if she's actively upset about whatever your husband did (not placing any blame), it might not be received well, in which case, I don't know that it would be appropriate. This is a little sticky. I think you should.