Given that Syana is a Bard with a decent +12 in Knowledge (the planes), she's most likely got a fairly good suspicion of Isera's true nature.

That said, there is a fair resemblance between the two (both have similar hair and eye colors, and Aeric has wings much like Isera's -- indeed, they are grafts, taken from a male erinyes), so she may suspect Aeric, as well.

Neither are open about her being a fiend, and they aren't going to directly confirm it if confronted, but they aren't going to explicitly deny it, either.