When I've sent Christmas cards after a death in the family or when a family member has been diagnosed with a terminal illness, I'm careful about what card I send.
I try to avoid ones that say 'Happy Holidays!' or 'Merry Christmas!' I found ones that said, 'Wishing you peace', which I felt was appropriate to the season, as well as the situation. And I would write something positive, if I could. 'I hope you have a good Christmas with your daughters', for example. I like Joy's wording, too.