Adding vault to family account after the death of someone

You cannot add the standalone account of A to your family account. It is a standalone account and stays this way. If you add A to your family account, you're creating a new and empty member account within the family account that might have the same name or email as login as the standalone account, but it's a different account.
Someone in the family account is never able to see any vault from any standalone account, except if he is signed into both at the same time.

Since you didn't write what you want to achieve in the end, it's difficult to say what you can do.

I guess you want to "inherit" and save the data from your deceased family member, so it doesn't get lost.

In this case, I propose you use the desktop app and sign in to both your family manager account as well as into the standalone account of your diseased family member. Now you're seeing all vaults from both accounts and are able to copy items back and forth. Create a vault in your family account and copy items from the vault(s) of the standalone account of your diseased family member into that new vault, so the items are transferred into your family account. Use the desktop app, it's able to multiselect and bulk copy items.
Then you can sign out from that standalone account and retire it. Set access control to that new vault according to how people in your family should have access to the imported items.

In case your family member used a v7 client with local vaults, you first need to copy the items from any local vault to a cloud stored vault first, otherwise you cannot access these items.

In all cases, you need the credentials (login name, account password, and in case of a new sign in the secret key as well) of the standalone account of your diseased family member to get access to his data. If you don't have that, the data is lost and you cannot gain access under any circumstances - not even 1Password can provide it.