I understand that some would want to take advantage of it, hence why I suggested that this could be bound to a few possibilities:
1st - proven relationship - it is not illegal to prove your relationship, it is only illegal to store such data, it's same as with banking, crypto wallets and otehr financial pages, where you have to prove your identity.
2nd - connection only under the same IP adress, this would prevent to sell such an account to others, as your family would count only to those living under the same roof.
I know it would be a lot of work, but it might be worth some brainstorming

Also about limitation, I think 3-4 should be somewhat a max amount that can be bound as family.
But hey, just thinking out loud
