I just don't see the secret being that they are siblings... Even half-siblings. I just don't see Emily and Melissa shrugging about potential incest. Nor do I see Erynn being calm about that fact.
And it isn't going to be that Melissa and Emily got together, not the way it is phrased.
What could be a secret, impact both Erynn and the MC, and be something that Erynn already knows, is okay with, but might make the MC leave again?
If it would make the MC leave again and has some impact on Erynn it can't be something about Andres. The MC wouldn't want to leave and go back to the person the secret is about and why would that have any bearing on Erynn? In addition if they were siblings or it would make the MC move away again wouldn't Erynn care? IIRC she took it well.
My crazy theory would be that maybe the MC does have a sibling that Erynn knows. Perhaps the Incest tag comes from the fact that Alexis is the MC's half sister. Emily doesn't recognize her because she's not met adult Alexis yet, she just knows that the girl that Erynn used to hang out with is Andres bastard child and the MC's half-sister?
(that's a pretty crazy theory though, I still think that Alexis is just the girl that Erynn knew as a kid and that's all)
I am curious about Erynn's dad... The lack of a mention so far is interesting. He is someone we don't know a thing about, Alexis' dad has more of a "presence" because of the car that she "borrowed".
Man, I wish Nyx wrote faster...