Hi! I have been researching Greek Mythology, specifically any and all characters pertinent to the Hades and Persephone tale. One tale says the river Styx was the mother of Persephone, and after further research, I read the Styx is sometimes identified as

There’s no such thing as too much curiosity ;)

The only reference to Styx being Persephone’s mother is in Pseudo-Apollodorus, which simply states:

“Styx bore him [Zeus] Persephone” (Bibliotheca 1.13)

…which was written much later in the Roman era.

Other than that, she is the daughter of Demeter. I suppose that would be an interesting take considering that Persephone is a Chthonic goddess. There aren’t any mentions of her being the child of Styx alone. But you are correct, Styx is also identified with Demeter Erinys, or “the wrathful earth”. So in that sense she’s also identified with the Erinyes.

I can’t really determine how or why this is, but the lineages of the gods are complicated by what tradition is followed, whether it’s Homeric, Hesiodic, Orphic, etc.