At least you got something ya know? I'm sure it wasn't that she wasn't thinking about you, she's your sister after all. Just she was probably not being organized is all. I don't think she had a malicious intent with it.

While, it was inappropriate and a different card would have been better, like just a 'how are you doing one' and an apology for forgetting the xmas one, she is sick so give her a little slack. ^_^

Hope you can sort it out without an argument. ^^