Most travel insurance sold these days have the stipulation that you have to purchase the policy within 15 days of making your initial deposit in order to be covered for cancellations due to pre-existing conditions in yourselves or your immediate family. There is also a rider available, also within 15 days of cruise deposit, that will cover some percentage of your non-refundable costs if you cancel for a non-covered reason.
Insurance policies cover you in the event that someone dies, whether or not it's from a pre-existing condition. Since your husband would be one of the ones taking out this insurance policy, your cancellation would be covered if the worst happens, because his sister is his immediate family.
It get trickier, however, if you have to cancel because if she takes a turn for the worse and you don't feel you can leave, since that WOULD be due to a pre-existing condition. I agree that you should call one or two insurance companies and clarify your options at this point.