Okey dokey, I'll wade in to this one.

Being close to my parents (and grandparents) I have on occasion flown 10+ hours on about 12 hours notice to go and sit at a hospital. I would personally cancel if at all possible and schedule down the line. You're always able to take time off, you may not always be able to have your wife be with her mother (especially since you have stated that she has cancer).

With that being said, it really is a gut check. I would support your SO in whatever way SHE feels is right. This is not about you right now.