@joko, My understanding is that when a person registers as an organ donor, it is honored as a legal agreement upon his/her death. He/she has already made the decision while still alive, and thus it will be valid upon death. However, the family/loved ones, are always informed of the persons decision before the donor process is carried out.
My friend wants to donate her body to science, and I remember her telling me that she is already registered with a particular medical facility to do that and her family is aware and in agreement. She said that has to be pre-arranged.
An interesting thing happened just 2 weeks ago when I was at lunch with this particular friend and a few others. We were discussing the new driver licenses and sharing our awful photos. I noticed that she did not have the Organ Donor notation on hers. That bothered my, until I remembered our earlier conversation.

@joko I am a registered donor, and have been for many decades. My driver's license has the indication on it, my advanced directive includes the information also plus my identification number for the donation service.

In my humble opinion, if a person has made the decision to donate their body or available organs upon death, that should be honored by family, no matter their own personal feelings. If a person expresses the desire to donate but does not formalize it, it then becomes the family's decision to act upon spoken word, or not. And sometimes, the family may not have anything formal, but knows that the deceased would have wanted to help others out, and makes that decision to set aside their grief in order to help others with donation of tissues/organs from their family member. It is truly a gift of life.
Ginger