Elektra was the earliest Okeanids and subsequently viewed as a second generation. This is mentioned in Hesiod's Theology. In the Preface of Hyginus' Fabulae it mentions that she was the mother of the goddess Isis as well as the Harpies Thaumas.
Okeanis & Tethys
Spouse & Lovers
"Also she brought forth a holy company of daughters who with the lord Apollo and the Rivers have youths in their keeping -- to this charge Zeus appointed them -- Peitho, and Admete, and Ianthe, and Electra, and Doris, and Prymno, and Urania divine in form, Hippo, Clymene, Rhodea, and Callirrhoe, Zeuxo and Clytie, and Idyia, and Pasithoe, Plexaura, and Galaxaura, and lovely Dione, Melobosis and Thoe and handsome Polydora, Cerceis lovely of form, and soft eyed Pluto, Perseis, Ianeira, Acaste, Xanthe, Petraea the fair, Menestho, and Europa, Metis, and Eurynome, and Telesto saffron-clad, Chryseis and Asia and charming Calypso, Eudora, and Tyche, Amphirho, and Ocyrrhoe, and Styx who is the chiefest of them all. These are the eldest daughters that sprang from Ocean and Tethys; but there are many besides."
- Hesiod, Theogony 346