I just want to pop in and stress again that "umbral aether" and "Light" are not the same thing and debating under that premise closes a gap within which important nuances might or might not lie. In the transcripts and game materials that I've read/saved/remember so far: Number of times explicitly umbral aether / umbral energies are mentioned in the same breath as sin eaters: 0

Alisaie
...A colossal wave of pure Light, drowning everything in its wake. Only Norvrandt was spared.
For the most part, living things are composed of aether of various different aspects.
But when exposed to such a flood, their aetheric harmony is shattered, and their natural form breaks down.
Then they either perish...or are warped into mindless abominations.
Yes, that's how the sin eaters came to be. They were once living creatures─or people─that were caught in the path of the Flood.
Once the change is wrought, there is no going back. In that instant, they are gripped by an insatiable appetite for aether, and will happily gorge themselves on any living thing within reach.
But even that is not the worst of it. The stronger sin eaters can plant Light in us like seeds in soil, corrupting our aether, and triggering the birth of new monstrosities.