The closest ports to your preferences are Charleston, SC, or Port Canaveral FL, or Baltimore. I mention driving because it a good way to save money.
This is my advice - but keep in mind I know nothing about you or your parents. A qualified travel agent can judge the real situation the best.
The only problem with Charleston is that the only ship that sails from there is older and the cruises are short, so you don't get "really" tropical. If you really want to give them something special arrange to drive them to Port Canaveral and give them a cruise to the Caribbean, east or west.
If they like snorkeling & other water sports the west is probably better, if they like shopping then the east Caribbean is probably better. But either is fine for both, really.
For the best selection of ships & itineraries Ft Lauderdale or Miami are the best bets, and from your proximity a plane ticket to Fort Lauderdale might not be too much. Why not ask your 5 siblings to help you pay for the cruise so it can be a gift from all of you?