We ran into a similar situation 9 years ago when we traveled to Europe (by land, not a cruise). My wife's brother was, unfortunately, diagnosed with cancer. He has since passed, but at the time, his prognosis was fair to good.
We insured our trip via InsureMyTrip.com. We were afraid we might have to cancel at the last minute if her brother worsened. The agent at InsureMyTrip was very helpful. But it was her opinion, and only an opinion, though an informed one, that if we were to do so, normal trip insurance would deny a claim due to pre-existing conditions.
Our solution, at her suggestion, was a policy known as "cancel for any reason" insurance. It's pricier than a normal policy, to be sure. And IIRC, it only reimburses 75% of the cost. But it was good peace of mind for us.
I hope this helps you.