I do think this is difficult, when one doesn't know the family well. When my last surviving parent died, I went through the address books (the one by the phone and the one in the Xmas card box) and made sure that everyone was contacted. Even so, the following Xmas, 2 cards arrived from people I hadn't managed to contact. I wrote and told them, with an explanation.
I tried to do this because over the years, I have had friends 'drop off' without anyone contacting me and it is very upsetting.
In a somewhat different situation - but one where I hadn't 'said goodbye' properly I did this: I went to a place that reminded me of my friend, sat on a bench and thought about her. I chose a card from a nearby shop, and sent it, with a cheque, to a charity that I knew would have pleased her. I posted it, saying 'thank you for being my friend'.