Yeah I really can't make a call on this one without actually knowing what she said. Like I feel like you're intentionally providing meaningless details, such as your sisters age and your wifes cancer, and leaving out the one important one - what was said.

Just what I thought - if the wife had been a jerk in her comment, no wonder the sister took offense, and recovering from cancer is no one's free pass at being a jerk to others. Although, calling the parents is a bit childish...