Maybe her husband has his own battles with her when you are not around, kittylester and can't face being a moderating force on your behalf. It's sad isn't it, you just want to be a loving family and support her whilst she is struggling. If it were me, I'd try to hold on to that apology whenever I felt sad or scared of her reaction.
One of the things that does stand out in your post, if you don't mind me saying, is that "She knew from the start it was caught early and her life was not in danger." Although I am sure you don't mean it, it sounds like you are dismissive of the cancer. Unfortunately, cancer sufferer worry that there is more they haven't found or they may get it again. That can be a drain on your emotional resilience.