maybe you could build it something simular to linux's permissions and make each family a 'group'. you then make each family member belong to said 'group'. then, when locking down certain sections, just check the 'group' has sufficient access.
that's pretty much what I have, each family member is linked to a family account.
The problem is not handling the accounts once created, the problem more related to the initial account signup. I've seen some similar systems use an invitational type system where one member can invite another, but i'm not sure that has the security i need.
here's a quick scenerio:
basically i need dad to be able to signup to the same family account as mom. once joined they will be billed seperately and cannot delete the others account.
but dad can only join the same account that mom has created (unless he wants to start his own family account).