Ok, so the more I thought about this the aunt didn't actually pay for 2 berths; she paid a 100% solo supplement WIHTOUT the port fees on the 2nd booking. Solo supplements can range from 0 - 100% (or more in some odd cases), but they do not represent 2 people. It is a penalty.
Taking the OP's logic about purchasing 2 seats in the YC for dinner, what's the difference if my DH wants to stay in and watch Monday Night football and order pizza and I go to the YC dining room and take a non YC friend? Isn't that the same logic? I paid for 2 seats and since he's not coming I can invite anyone I want, right? (Or if his pizza counts as his meal, let's say he doesn't feel like having dinner at all, so I can just substitute someone else for him in the YC dining room by this logic, and he actually PAID for a place at the table....)