Midrasz do Diwrej ha-jamim I 1:38

He concerned Himself also with the genealogy of Esau’s sons in the chapter These are the generations of Esau to disclose their degeneracy. You find that they too were the offspring of incestuous relations. Scripture states in one place: And the children of Elephaz were: Teman, and Omar, Zepho, and Gatam, and Kenaz. And Timna (ibid. 36:11), and elsewhere it states: And Timna was concubine to Elephaz (ibid., v. 12). This clearly informs us that Elephaz had sexual relations with his own daughter. How did that occur? He had sexual relations with Seir’s wife first and made her pregnant and begot Timna. Later he married Timna, as though she were Seir’s daughter, when in fact she was his own. That is why it says: And the children of Seir: Lotan … and Timna was Lotan’s sister (I Chron. 1:38–39). She was Lotan’s sister by her mother, but not by his father; Elephaz was her father, and Timna became the concubine of Elephaz the son of Esau.