Request a vehicle letter – Affidavit

In another post I have already told about a Golf VI that I get about acquaintances. Unfortunately there are problems with the vehicle letter – about the process of the reapplication I am quite confused. The situation looks like this. In the family of my girlfriend an elderly (and ill) family member had a car – which could not use the vehicle anymore because of age and illness. Now the car was newly registered and brought by me to a friend’s workshop. I have received the vehicle certificate (declared “declared” by the registration office) and unfortunately no one can find the vehicle letter. However, I need the letter now for the registration or registration. The application for a vehicle letter seems to be a rather difficult matter – but I have found contradictory information. On the one hand, I read that only the original holder can provide an affidavit, to apply for the letter. Unfortunately, the original holder is not able to provide an affidavit to the admission office. Is the only alternative really the signing with the notary? I would like to avoid this because of the expenses/costs. Or is the completed form sufficient for the signing of the affidavit insurance, as it can be found on the website of some admission offices? The owner’s brother has a general power. Is that enough for the affidavit? Is there also a way to keep on annoying the family without the issue? Would a sales contract suffice in which it says that the vehicle letter has been lost? Thank you for your help.