Yeah, Voidsent aren't necessarily all mutated by Auracite. Especially not because Voidsent aren't only people, but animals as well that were corrupted.

Whether or not the Auracite was actually the reason for the mutation, isn't certain. It's possible that when people with Auracite started fighting each other for more power, it created a bunch of Dark, and this Dark is what caused the mutations during the flood of Darkness that would engulf the world.

If it was in fact, the darkness that caused the mutation and not the Auracite (Also, if this is how Voidsent such as the ones in Haukke Manor (Hard) turn Wood Wailer's blood into Voidsent, via infusing them with Dark - Which I believe is the case, given that it doesn't state that they "Summoned" a Voidsent to the realm, but "Turned the Wood Wailer's blood" into one) then it would stand to reason that a similar method created the Sin Eaters, with the Flood of Light creating high amounts of Light that corrupted living beings and turned them into Sin Eaters.

Though, how the hierarchy works, is a mystery. Perhaps the hierarchy of Voidsent is that beings that were empowered by the Auracite became the higher ranked Voidsent. Though if that is the case, then we'd be left to wonder about the hierarchy of the Sin Eaters, since, as far as we know, people on the First weren't using Auracite. I suppose we could theorize that people whom are exposed to high levels of light throughout their life, such as those heroes whom had been toppling Primals, would be more susceptible to the corrupting nature of pure Light and thus become the higher ranking Sin Eaters.