From what i know, all you need is a medical note and you should get full refund
Not correct, if you didn't purchase the insurance there may be penalties depending on the sailing date. Cruiseline insurance must be purchased prior to the penalty period. Some outside insurers provide insurance policies that can be written even after the penalties begin but have exclusions for pre-existing conditions and other circumstances.
The penalty period normally begins 60 days prior to your sailing date on short sailings and 75 days prior for 7 night sailings. Some promotions, like SWAT and FLASH promos require payment in full at the time of the booking and are 100% penalty from day one (Carnival doesn't do these promos so you're okay there).
If you are in penalty and your father-in-law is already in poor health you're probably wasting your mony to buy insurance at this point. The insurance provider has the right the medical records in order to approve a health related claim. This will fall under the pre-existing condition exemption and would result in a denied claim. If you're still outside penalty buy the insurance through an outside source and be certain the policy you purchase covers pre-existing conditions, many do.