A coworker’s niece is battling cancer and we’ve been asked to send her Christmas cards early. Cheer her up. It’s not looking good. I’m not sure how long she’s got left. 
Should I say anything at all about her illness? Encourage her to get well? Or just keep it light? I got a humorous Christmas card.
I just don’t know what to say. I don’t want to sound cliched with a hang in there or get better remark. This matters.
You shouldn’t not mention it, then it becomes even more noticeable! I would say something like
“Dear X,
I was saddened to hear of your illness from ____. I wanted to say that we are thinking of you this Christmas season. We hope you have a wonderful holiday and enjoy yourself.
Merry Christmas and wishing you well,”
Tweak as you like!
Thank you Anaamika
advice noted and taken. 
I’ll print too just in case she wasn’t taught cursive. She’s just a teen.