Yes, I know, another Darkin Theory (sorry)
What if Sona's etwahl is a Darkin? I mean, she can kill people with it, and possibly an accident with it or something is why the orphanage took it away, only for it to reappear for Sona? Plus, if she was adopted by a rich Demacian who heard about it, a rich woman would have the access to the education that would familiarize her with what a Darkin was.
If Sona has a Darkin weapon, that almost gives us a full Darkin team, now we just need a mid laner. Hmm...