The Gemara discusses which laws the Noahides and later Avot were obligated in before the Sinai revelation, implying that the full Torah corpus was not yet binding on the patriarchs.
תָּא שְׁמַע: ״וַיִּקַּח עַמְרָם אֶת יוֹכֶבֶד דֹּדָתוֹ״. מַאי לָאו, דּוֹדָתוֹ מִן הָאֵם? לֹא, דּוֹדָתוֹ מִן הָאָב. תָּא שְׁמַע: ״וְגַם אׇמְנָה אֲחֹתִי בַת אָבִי הִיא אַךְ לֹא בַת אִמִּי״ – מִכְּלָל דְּבַת הָאֵם אֲסוּרָה. וְתִסְבְּרָא אֲחוֹתוֹ הֲוַאי? בַּת אָחִיו הֲוַאי! וְכֵיוָן דְּהָכִי הוּא, לָא שְׁנָא מִן הָאָב וְלָא שְׁנָא מִן הָאֵם – שַׁרְיָא. אֶלָּא הָתָם הָכִי קָאָמַר לֵיהּ: קוּרְבָּא דְּאָחוֹת אִית לִי בַּהֲדַהּ, מֵאַבָּא וְלָא מֵאִמָּא.
Come and hear a proof for the opinion of Rabbi Akiva from the verse: “And Amram took Jochebed his aunt as a wife” (Exodus 6:20). What, was she not his maternal aunt? Presumably, Jochebed was the sister of Kohath, Amram’s father, from both of Kohath’s parents, and not from his father alone. Evidently, a descendant of Noah may marry his father’s sister. The Gemara rejects this proof: No, she was his paternal aunt, Kohath’s half sister. Since she was not Kohath’s sister from his mother’s side, she was not forbidden to Amram. Come and hear a proof for the opinion of Rabbi Eliezer from what Abraham said to Abimelech with regard to Sarah: “And moreover, she is my sister, the daughter of my father, but not the daughter of my mother; and so she became my wife” (Genesis 20:12). By inference, the daughter of the mother of a descendant of Noah is forbidden to him. The Gemara rejects this proof: But how can you understand that Sarah was Abraham’s sister? She was his brother’s daughter. By tradition, it is known that Sarah was Haran’s daughter Iscah. And since that was so, there is no difference whether they were paternal relatives, and there is no difference whether they were maternal relatives; in any event she was permitted to him, even according to the halakha of Jews. Rather, this is what Abraham was saying to Abimelech there: She is related to me like a sister, as the daughter of my brother is like a sister, and our relationship is from the side of my father but not from the side of my mother.