Nonnie, so sorry about your loss. I can understand why you feel upset and cross after receiving this thoughtless Christmas card.
You say you are thinking of writing to them explaining how you feel. I think that's a good idea, but don't send it, you may find that you regret it if you do. Instead write away, put everything you feel about them down on paper and then throw it away, or burn it, I hope it will help you find peace without any nasty aftermath or regret that they have made you act out of character, it has certainly worked for me in the past.