Hi Polphi
Welcome to cruise critic.
As Schoifmom has said, your family member "might" be able to get away without having to pay any penalties if you just don't show up. Again, as said, do not cancel. The key is that they may have to pay full price for both persons.
The main problem is that this promotion is only allowed to be used by the person that it was addressed to, and also it is stated that you were required to sail. Your family member cannot replace you without paying full current price for anyone who sails.
We all get promotional offers, some better than others, we cannot trade these offers. If it was as easy as you had hoped, everyone would do it. Maybe someone who has personal experience will chime in, but I wouldn't be too sure of it happening without extra cost, especially if you contact them to remove your name from the booking.