Good points. I’ll be more explicit about the details:
If, at the time of formation, you don’t know which communities would be siblings, then it’s the same as the current status quo, so I don’t see that as a comparative disadvantage. In any case, there’s no reason to rush into siblinghood. One hope would be that the existence of the term “sibling community” itself would encourage people to discuss possible connections, even when they’re not yet connected. I hope it brings like-minded groups together.
The sibling relation would need the consent of both mod teams, not just one side, so it can be unilaterally severed, but only jointly formed. No one would force lefty news and righty news to become siblings. But there are currently 5+ major “Technology” communities that are almost entirely overlapping. I hope siblings would allow them to overlap where appropriate but maintain their unique identities.