There are a few open topics in the forum about this. y recommendation is that all the information is stored in a file and then password protected. Then you create 3 copies of the file and give them to three people you trust. Lastly divide the password in two and put these two pieces in two different bank safes. You may want to use 4 safes to have two copies.

If the amount you have does not justify these expenses, you can just give parts of the key to trusted people or to 2 lawyers, notary public or similar.