I know you're looking for a definitive answer, but It may not be possible at this very point considering your question requires things that haven't been completely fleshed out yet. I do believe the want is for people in your situation to be able be in the same family, however the exact method on how to do so hasn't been determined yet. I hope these Q&A's about Surname Reservation help.
Source
Q18: Can I claim the same Surname, and be a part of an extended family?
A18: We're working out the details and will have more to share in the future. For now, claim the surname you want
Q22: May I choose my own surname now even if I am planning to play inside other family?
A22: Sure, it still puts that surname on the server and allows you to create characters with it at any time.
Q26: I want my friend to enter the same surname as I did so we can start out in the same family, but it won’t let us do it because I’ve already claimed it. What can we do?
A26: The reservation tool works as intended. Surname reservation does not allow you to join the same family that way. Stay tuned for more information on personas and how to become part of the same family at a later point in time.
Q37: If players intend to play as a family with the same surname, or two counts intend on playing as brother and sister, can they all choose the same surname?
A37: They can be in the same family, which is why "surnames wont always be unique" is accurate. If a group of players want to be part of the same family, there will be a way of the surname owner to share that last name with them with personas.
I hope these to some extent help, the best answer I can give is that SBS seems to want players in your situation to be able to do what you want to do, the exact method on how to do it however hasn't been publicly announced or perhaps figured out yet.