There are women who would laugh it off and possibly agree with their spouse. There are others that would be upset by this brutal honesty. It’s about how your sister handles this opinion that matters…not yours or mine.
As angry as you might feel about the BIL’s comment, it doesn’t help your sister out to share your feelings unless she asks for your advice. Right now, she has more important things, like her health, to focus on rather than a comment that offends you.
With that said, it is understandable how you feel. Your three days with her and the laughter was exactly what she needed. Hang in there dear friend. Cancer and the treatments means a roller coaster ride for the whole family. I’ve been in your shoes, and my heart goes out to you.