This happened to me at the Atlanta Marquis. My sister and I were on a reservation for two rooms. One was a points redemption. The other was a cert. They would not let her check in to the cert redemption because it was only valid for me. Ultimately, she checked in with her credit card and when I got there an hour later it was switched back to the cert. I did not realize, though, that a seven day cert package was handled this way.
Members try to game the system by putting in a second name and having the second person check in and say that the named member will be showing up later, but this is gaming the system. Once the hotel figures out the named member will not be arriving, the hotel does not have to honor the cert. Either this hotel is being vigilant or your family member said something to arouse suspicion.
Your best bet would be to call Marriott and see if they will reissue the cert in your family member's name. The cert associated with the travel package appears to have been issued in your name. The hotel isn't going to waive rules for you but maybe Marriott will help. I don't see anything in the T&Cs that requires the cert be issued in your name, but obviously it has been. Good Luck.