Styx is often referred to as Oceanus and Tethys’ eldest daughter, but do you know a source stating she is his eldest child? Narrative-wise it makes sense she is his favorite by seniority, but I think some of the Potamoi are older than her, since we have Asterion, a river god, whose children became Hera’s nurses
Also a bonus question if you know: Do River-goddesses and Oceanids have a distinction? Theoi.com does a thing of placing Lethe at the Potamoi section and not at the Oceanid section, since she is technically a river, but the Potamoi are notably called “sons” (unless Lethe was actually trans, which isn’t possible at all by old accounts but fun to think about)
I only know of her being oldest of all the daughters of Okeanos, as Hesiod calls her: "terrible Styx, eldest daughter of backflowing Ocean." (δεινὴ Στύξ, θυγάτηρ ἀψορρόου Ὠκεανοῖο πρεσβυτάτη), πρεσβυτάτη being a world that can mean both eldest in age or most revered, most honoured.
Personally I think that the best contenders for the title of eldest child of Okeanos are Styx herself and Acheloos. She because Hesiod says that Okeanos gave her a whole tenth of his waters and it makes sense for me (and feels less awkward) if he would have done so out of enthusiasm for the birth of his first child and since he did not yet have other offspring to consider; him because Acheloos seems to have been considered an especially important river, greatest/mightiest after Okeanos himself, as in this passage from Iliad 21: "And as Zeus is a greater god than the gods of the sea-bound rivers, so is his offspring greater than a river’s child. Now a great river washes your feet, but cannot save you, for none can fight a scion of Zeus. Not even Achelous vies with him or the mighty and deep-flowing Ocean, source of the rivers and the sea, the springs and the deep wells: even he fears Zeus’ lightning, when dread thunder crashes from the sky." His name is sometimes used to refer to water in general, in the Derveni Papyrus Zeus recreates Okeanos and after him Acheloos, and it seems that the historian Ephoros of Kyme and the mythographer Akousilaos of Argos referred to this river as most ancient and honoured among the 3000 sons of Okeanos. I guess Acheloos and Styx could be twins, why not?
As I discuss at more length elsewhere, I believe Okeanos and Tethys to be the eldest Titans and to have started having children long before any of their other siblings did, but even without that interpretation I find it quite likely that many of their river sons and Okeanid daughters are actually closer in age to their aunts and uncles than to their cousins. So Asterion (assuming that he is a son of Okeanos) being younger than Styx and nevertheless being the father of Hera's nurses would still make sense to me.
River goddesses seem exceedingly rare. I can't even think of any others besides Styx and Lethe. Female water divinities usually seem to be springs, wells and the like, or to have their dwelling in the sea in the rarer instances when they aren't deities of fresh water. So the major difference between river goddesses and Okeanids is that the latter are in their vast majority not rivers, with the notable exception of Styx (since Lethe is never as far as I know called a daughter of Okeanos).
