I just reread the chapter 'Strength ' and it struck me.... wouldn't Navani be the perfect person to bond the Sibling? She literally thinks "She had no problem being a wife and mother to monarchs, but to be one herself -- storms, what a dark path that would lead them all down."
She is also the one who's very interested in Fabrials, we get a reminder how she's dealing with getting Urithiru to work on a day to daily basis... And if the hints we get about the Sibling are true, its Bondsmith is bound to the Tower. With other fabrials... the spren is always the one bound, but with the Sibling... the human is bound to one place as well. And who knows what being bound to the Sibling and running the city might do to one's psyche, or physique (I'm getting Babylon 5 Machine flashbacks now). Whomever bonds the Sibling would thus have to be someone who is willing to make this sacrifice but also capable of understanding what it takes to run a city-state. And this chapter... imho it does suggest Navani could do this.