The Sibling is, currently, the spirit of Urithiru, and the pseudo-Fabrials in it are expressions of their body.

My question is, was the Sibling created at the same time as Urithiru? Or was the Sibling created at the same time as Stormfather and Nightwatcher, and incorporated into Urithiru as it was built? Or, for that matter, could Urithiru have been build first, and the Sibling have emerged organically from the cognition of the people living in it, the way other spren are?

Maybe not related, but Venli's Stoneshaping visions insinuated that there was a ancient Singer city here in the extreme past. could the Sibling have been somehow tied to this city too?