I planned for there to be Sirens for each race as appropriate to come collect their respective race, and because they're just souls that live in Sanctuary and are chosen to go collect other souls and return immediately so Hum doesn't have to be in multiple places at once and so Hum can still do his job of collecting souls from the Natural World despite being incentivized to remain almost pernanently in Sanctuary. They're not specifically a race or creature, more a job Hum gives out that has no real impact on the Natural World where things really matter. Hum can twist the lands of Sanctuary at will without expending Power, it makes sense he can just delegate soul collecting to some group of souls already in Sanctuary. But I am interested in theo's opinion.

And other Gods have zero power over Sirens as far as I can tell. They're souls in Sanctuary, Hum is lord of Sanctuary which is an entire Realm on its own. Other Gods can't Create Order, Command Race, Advance Race on Sirens and Sirens aren't a civilization on their own for instance.

If we wanted to be super technical it could be said that by Hum doing his job and "leaving" Sanctuary to collect souls, he isn't "remaining" to accrue the extra 3 Power. But that seems to me to not make sense, so Sirens are a simple solution.