Midras Kejadian 20:12: Bereshit Rabbah

“Therefore, a man shall leave his father and his mother, and he shall cleave to his wife, and they shall become one flesh” (Genesis 2:24).
“Therefore, a man shall leave” – it is taught: A proselyte who converted while married to his sister, whether [the relationship is] paternal or maternal, he must divorce her; this is the statement of Rabbi Meir. The Sages say: If she is his maternal [sister] – he must divorce her, but if she is his paternal [sister], he may keep her, as there is no paternity for idolaters.³⁰According to the Sages, the halakha does not recognize a connection between a person and his father if they are “Noahides,” that is, non-Jews. They raised an objection to him [Rabbi Meir]: But is it not written [regarding Abraham and Sarah]: “And also, indeed, she is my sister, the daughter of my father, [but not the daughter of my mother, and she became my wife]”? (Genesis 20:12).³¹The implication is that it is permissible for a Noahide to marry his sister from his father’s side, but not a sister from his mother’s side, in accordance with the view of the Sages. He said to them: He [Abraham] answered them³²The Philistines, to whom he was speaking. according to their own opinion.³³In Philistine culture this distinction was made, but not according to halakha. Rabbi Meir raised an objection to them: “Therefore, a man shall leave his father and his mother.”³⁴This is understood to mean that a man may not marry his relatives, and it mentions both mother and father. Rabbi Yoḥanan said: They explained it to him: “Therefore, a man shall leave his father and his mother” – one who is related to his father or related to his mother.³⁵One may not marry his aunt, whether it is his father’s sister or his mother’s sister. But the definition of sister is still only a maternal sister.
Rabbi Abahu raised an objection [to Rabbi Meir]: But is it not written: “Amram took Yokheved his aunt [as his wife]” (Exodus 6:20)?³⁶Yokheved was the sister of Kehat, Amram’s father. Yokheved and Kehat had a common father, Levi, but they may have had different mothers. Rabbi Shimon son of Rabbi Abahu said: If it is so [as Rabbi Meir says], before the giving of the Torah Israel did not observe the law even according to the standards of Noahides? This is astonishing. Rabbi Levi said: They explained it to him: “Therefore, a man shall leave [his father and his mother]…” – one who is related to his father or related to his mother.³⁷This explanation was mentioned earlier, and is an exposition of the Sages’ view that the verse is referring to one’s aunts, but in any event only maternal sisterhood is recognized, not paternal sisterhood. The Midrash does not answer the question posed to Rabbi Meir concerning Amram; see commentaries.
Rabbi Abahu said in the name of Rabbi Yoḥanan: Noahides incur liability [for committing adultery] for [consorting with] married women but are exempt for [consorting with] betrothed women.³⁸If a woman is betrothed to a man, and another man has relations with her, it is not considered adultery for Noahides. Betrothal is not considered a marital relationship for Noahides.
Rabbi Yona said in the name of Rabbi Shmuel: A promiscuous woman³⁹An unmarried woman who consorts with many different men. who is standing in the marketplace, and two men had relations with her, the first man is exempt,⁴⁰He did not commit adultery. and the second incurs liability, because she is now “a woman who has had relations with a man” (Genesis 20:3).⁴¹This is the literal translation of be’ulat ba’al (Genesis 20:3), which is usually rendered “a man’s wife.” In any event, it is the definition of a woman who may not consort with another man, as that would constitute adultery. But did the first one intend to acquire her through intercourse?⁴²How can a woman be considered married as a result of a non-committal act of intercourse? This shows that regarding Noahides, intercourse with a woman [even with no intent of marriage] effects acquisition [of her as a wife], which is not in accordance with the law [regarding Jews].⁴³The law regarding Jews is that marriage cannot be effected without intent.
What is the law regarding divorce for them [Noahides]? Rabbi Yehuda ben Rabbi Simon and Rabbi Ḥanin said in the name of Rabbi Yoḥanan: Either they have no divorce, or each one must divorce the other.⁴⁴This is derived from “He shall cleave to his wife.” Neither partner may dissolve the union unilaterally. Rabbi Yoḥanan [later] said: His wife may divorce him, and she pays him double the marriage contract.⁴⁵According to all opinions, the divorce proceedings of Noahides are unlike those between Jews.
Rabbi Ḥiyya taught: An idolater who divorced his wife and she went and married another, and both of them⁴⁶The first husband and the woman. went and converted to Judaism, [and the first husband wants to remarry her,] I do not apply regarding him the verse: “Her first husband, who divorced her, [may not take her again to be a wife for him]” (Deuteronomy 24:4).⁴⁷The prohibition against remarrying one’s wife after she has been married to another man in between applies only when the first marriage was dissolved by a divorce recognized by halakha, but marriages among Noahides are not dissolved by a halakhic divorce procedure, as was just explained.
Rabbi Aḥa said in the name of Rabbi Ḥanina bar Pappa: In the entire book of Malachi “The Lord of hosts” is written, but here [in the following verse], “God of Israel” is written, as it is stated: “For I hate divorce, said the Lord, the God of Israel” (Malachi 2:16). It is, as it were, that He associates His name [regarding divorce] only to Israel.
Rabbi Ḥagai said: When Israel returned from the [Babylonian] exile, the women’s faces had become darkened in the sun,⁴⁸On the long trek to the Land of Yisrael. and they [their husbands] would abandon them and go and marry Amonite women. They⁴⁹The Israelite women. would circle the altar and weep [about this cruel treatment]. This is what Malachi says: “This is the second sin that you do” (Malachi 2:13) – the second, after Shitim.⁵⁰There the Israelites sinned with the daughters of Moav (Numbers 25:1). “Covering the altar of the Lord with tears, weeping and moaning, [causing Him to no longer accept your offerings or receive them with favor from your hands … because the Lord is witness between you and the wife of your youth, whom you have betrayed]” (Malachi 2:13–14) – the Holy One blessed be He said: ‘Who could accept [offerings] from them, due to the weeping and moaning [that they caused to their wives]? After you robbed, stole, and took her beauty from her, you send her away? That is shocking.’
From where is it derived that it is prohibited for them [Noahides] to engage in illicit sexual relations? It is as it is stated: “And he shall cleave to his wife” – but not to the wife of another, not to a male, and not to an animal. Rabbi Shmuel, Rabbi Abahu, and Rabbi Elazar said in the name of Rabbi Ḥanina: A Noahide who has unnatural intercourse with his wife incurs liability for the death penalty. Rabbi Asi said: Any prohibition that is written regarding Noahides is neither a [mere] positive mitzva nor a [mere] negative prohibition, but one that incurs the death penalty. How does one arrive at this?⁵¹What Rabbi Shmuel et al. said. “And he shall cleave to his wife, and they shall become one flesh” – in the place that they can produce one flesh [a child].

“Avimelekh called Abraham, and said to him: What have you done to us, and by what have I sinned against you, that you have brought upon me and upon my kingdom a great sin? Deeds that should not be done, you have done to me” (Genesis 20:9).
“Avimelekh called Abraham, and said to him” – “what have I sinned against you?” “What have you done to us?” – this is [referring to] what it says: “You will die because of the woman that you have taken…” (Genesis 20:3). “What have I sinned against you?” – [this is referring to what it says:] “If you do not return her, know you that you will die” (Genesis 20:7). “That you have brought upon me and upon my kingdom a great sin [ḥata’a]” – “for the Lord had obstructed all wombs of the house of Avimelekh” (Genesis 20:18).²⁹The Midrash interprets ḥata’a to mean punishment (see Zechariah 14:19). “Deeds that should not be done” – Rabbi Ḥelbo said: [Avimelekh said to Abraham:] ‘Everywhere else [you went], prosperity used to go before you,³⁰That is, it used to accompany you. but [when you came] here, famine goes before you.’³¹Situations that did not usually occur elsewhere in your travels, have occurred here. ]That is what he meant by:] “Deeds that are not done [you have done to me].” [This paragraph is the explanation of the subsequent verse:] “Avimelekh said to Abraham: What did you see, that you did this matter?” (Genesis 20:10).

“Abraham said: Because I said: Surely there is no fear of God in this place, and they will kill me over the matter of my wife” (Genesis 20:11).
“And also, indeed, she is my sister; the daughter of my father, but not the daughter of my mother, and she became my wife” (Genesis 20:12).
“Abraham said: Because I said: Surely there is no fear of God…and also, indeed, she is my sister; the daughter of my father…” – he answered them in accordance with their practice.³²They permitted marrying one’s paternal sister but not one’s maternal sister. Thus, Abraham was explaining to them that his marriage to Sarah, according to the prevailing practice, was valid.
“It was, when God caused me to wander from my father's house, I said to her: This is your kindness that you shall perform for me; at every place that we will come, say of me: He is my brother” (Genesis 20:13).
“It was, when God caused me to wander [hitu] from my father’s house…” – Rabbi Ḥanin said: If only we could expound this verse in three ways and thereby satisfy our obligation.³³The verse contains two difficulties. First, the fact that the verb hitu is plural, as if there were many gods commanding Abraham; second, the fact that hitu indicates that God corrupted him, as the root of that word, ta’a, can mean either to wander away or to become corrupt. By proposing these following three interpretations, my hope is that I have fulfilled my obligation to address these difficulties. [Abraham was saying:] When the nations of the world sought to fight against me while I was still in my father’s house, the Holy One blessed be He protected me.³⁴The subject of the plural verb hitu, then is not God, but the nations of the world. The word God is expressing a separate thought; that God saved me from them. When the nations of the world sought to lead me astray [lehatot], the Holy One blessed be He revealed Himself to me and said to me: “Go, you” (Genesis 12:1). When the nations of the world sought to stray [litot] from the ways of the Holy One blessed be He, He appointed for them two prominent leaders from my father’s family, Shem and Ever, who would give them warning [to repent].