Many thanks for your replies.
I'm so relieved there's no problem me continuing to send a card to cousin's husband and children, particularly this first year.
I do agree with the subdued card so even though I usually buy a box of regular Christmas cards, I'll look our for a specific one for them.
A slight diversion but on the same theme: many years ago when my son was in hospital, we met a woman whose daughter was on the same ward. A couple of years later, we got news the child had died and for a few years after that, the cards we received from her mother would be signed from herself, her husband, her youngest child and the initial of the deceased child in a circle. I figured she wanted to still remember her daughter and I didn't know whether it was the right thing or not, but I mirrored what she did in my card to her as my way of saying I hadn't forgotten her daughter. When she stopped doing it, I stopped, too.