In general, I think it's difficult to combine collaborative custody with family custody.

This is, mainly, the reason why these companies allow you to add "trusted beneficiaries", so they can, in the worst case scenario, acquire the key that they hold for you.

If you wanted to do a solution where no 3rd party was involved, pooya87's answer is the best you 'll find, while there are some good variations that you could apply.

How does Unchained handle their keys with their customers?

Supposing you create a 2-of-3 vault, do they give you all the 3 keys, while simultaneously maintaining a backup of one of these themselves?

If so, then essentially transforming pooya87's solution into adding Unchained too, you could make a 2-of-3 multisig and:
1. keep the key A for yourself
2. give the key B to your inheritor X
3. allow unchained to hold a backup of the key C and also keep it for yourself.

Changing the key B would create multiple vaults and each vault would be given to your corresponding inheritor.

This would essentially improve pooya87's suggestion, in a way that, even if the worst happens, the inheritors would be able to obtain the key C from the company, so they wouldn't be locked out of the vault.