Some password managers provide an offline root of trust which family members can use in this scenario. For example, 1Password tells users to print off an "Emergency Kit" which is a physical piece of paper with secret recovery codes printed on it, which they store in one or more safe places. [1]

If someone passes away, their family members can use the Emergency Kit to gain access to and use all their credentials - including their passkeys.

(The Emergency Kit also allows you to recover your data in the event that you forget your master passphrase or lose all your devices.)

[1] https://support.1password.com/emergency-kit/