I don't quite understand your family's position with the marina. Do they own it? Do they lease it from someone and merely operate the business (the marina)? If it is the latter, the owner can seek out offers on the marina at its/his/her/their whim. That doesn't mean they can displace your family absent a provision in the lease that allows them to recover possession of the marina. Perhaps they want to see if there is interest in their property. If there is and your family's lease is not a long term obligation, they are merely hedging their position.
If, on the other hand, your family owns the marina and the company running the add does not have your authority to list it for rent/sale, then a quick letter to them demanding that the cease and desist would be appropriate. If that does not work, contact the Pennsylvania authority that oversees and/or licences real estate brokers. Either way, you should get their attention.