If you are not all on the same policy, and you (the passenger with the medical condition) cannot travel for any reason relating to your cancer, the other members of your party would not be covered for cancellation by their policy unless they had declared your medical condition on their policy. So it makes sense for you all to be on the same policy, then if one person cannot travel, the others would be covered for cancellation of the whole trip if they didn't want to travel without you. But note that you must read all the small print and check any terms and conditions that are important to you.