I recommend reading some of Pamela Morgan's posts about bitcoin inheritance:

What you are describing sounds to me a bit like a "family office" fund that happens to be invested in BTC, and you are looking for a relatively decentralized way for the family to manage custody of the BTC held in the fund. This might be an easily understandable way to introduce the idea to a lawyer if you seek council about this (and I recommend that you do -- I am not a lawyer).

Regarding wallet options, you could check out Nunchuk which is one of the easier-to-use multisig wallets and it is open-source which is nice from an auditability perspective. Electrum, Sparrow, and Specter are all good options as well but a bit more advanced usability-wise. Check them out and you can decide for yourself what is best for your situation. See also https://btcguide.github.io/ for general multisig tips.