Are sin eaters only able to see ghosts who are in twilight, or can they see other discorporated spirits or angels?
They can see ghosts without issue (Geist: The Sin Eaters p. 174):
Sin-Eaters see ghosts as readily as they see the living, and can easily discern between the two.
To see in the Twilight, they need to reverse possess their geist (Geist: The Sin Eaters p. 175):
A Sin-Eater can transfer his consciousness into his geist [...] This allows the Sin-Eater to see not just ghosts, but anything in a state of Twilight, as well as physically interact with its denizens and any Twilight objects.
This would lead to a situation where the Sin-Eater sees & interacts with all Twilight entities, but the different types of ephemeral beings do not affect one another. As found in the God-Machine Chronicle p. 215:
Instead of bodies formed of flesh and bone, ephemeral beings are made up of spiritual matter called ephemera, which comes in several varieties. These substances are both invisible and intangible to anything not comprised of the same sort of ephemera[...]
- Ghosts see other ghosts & nothing else
- Spirits see other spirits & nothing else
- Angels see other angels & nothing else
- Sin-Eaters see & interact with everything