Which brings me to the following problem: What should happen to them in case of my death or other circumstances where I am not able to provide the master passwords anymore?
What is your approach for your passwords in that situations as some of them might be of big interest to your relatives for certain services or to help you in certain situations.
Provided you have two or more relatives you can trust to not conspire while you are alive, you can make use of this system to cover for the contingency:
Shamir's Secret Sharing https://en.wikipedia.org/wiki/Shamir%27s_Secret_Sharing