Берешит 36:12 Мидраш: Midrash Tanchuma, Bereshit Rabbah & Sifrei Devarim

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.

“Timna was a concubine of Elifaz son of Esau, and she bore Amalek to Elifaz. These are the sons of Ada wife of Esau” (Genesis 36:12).
“Timna was a concubine of Elifaz son of Esau” – Rabbi Shimon ben Yoḥai taught: Why do I [need for the verse] to expound and say: “Timna was a concubine of Elifaz son of Esau”? It is to inform us of the praiseworthiness of the household of Abraham our patriarch, to what extent the kingdoms and realms wished to cleave to him. Who was Lotan? He was one of the rulers, as it is stated: “The chieftain of Lotan” (Genesis 36:29), and it is written: “Lotan’s sister was Timna” (Genesis 36:22). “Timna was a concubine…” She said: ‘Since I am not worthy to marry him as a wife, I will be his maidservant.’ The matters can be inferred a fortiori: If the wicked Esau, who had to his credit only one mitzva, because he honored his father, kingdoms and realms wish to cleave to him, all the more so that they would wish to cleave to the righteous Jacob, our patriarch, who fulfilled the Torah in its entirety.

(Devarim 32:47) "For it is not an empty thing for you; for it is your life, and by this thing you shall prolong days": There is nothing empty in the Torah, for which, if you fulfill it, you will not be rewarded in this world, with the principal remaining for the world to come. Know this to be so, for they said: Why is it written (Bereshith 36:22) "And the sister of Lotan was Timna," and (Ibid. 36:12) "And Timna was a concubine to Elifaz (the son of Esav)"? Because she said: "I am not worthy of being his wife; (at least) let me be his concubine." Why all this? To apprise us of the greatness of our father Abraham, kings and sultans [(Timna was a daughter of royalty)] desiring to marry into his family. Now does this not follow a fortiori? If Esav, who kept only one mitzvah, that of honoring his father — kings and sultans desired to marry into his family, how much more so would they run to do so with Jacob the tzaddik, who fulfilled all of the mitzvoth, viz. (Ibid. 25:27) "And Jacob was a whole man!"