FY, I think your wife's comments about your sister's cancer can be taken another way. Please keep in mind that these are your wife's thoughts/comments as of right now and we do not know how she will feel in months to come. Your wife may be looking at the entire picture a bit differently than you. She most likely is thinking about all of the worry, testing, surgery (if needed), chemo, etc. It not only takes a toll on the patient, but on the family as well. Her comments give me the feeling that she doesn't want to have to worry about a diagnosis such as this and she would be better off not knowing and living her life to the fullest to the end. I can understand how she feels about it because I've seen so many go thru the cancer diagnosis, chemo, etc., and what it does to not only the patient, but the family as well.
If she truly feels this way about herself, her body and her life, then she may not change her mind even if she sort professional help. The best approach is to casually ask her why she feels the way she does and see what she has to say.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.