אחתי בת אבי הוא. וּבַת אָב מֻתֶּרֶת לְבֶן נֹחַ, שֶׁאֵין אַבְהוּת לְגוֹי, וּכְדֵי לְאַמֵּת דְּבָרָיו הֱשִׁיבָם כֵּן. וְאִם תֹּאמַר וַהֲלֹא בַּת אָחִיו הָיְתָה? בְּנֵי בָנִים הֲרֵי הֵן כְּבָנִים, וַהֲרֵי הִיא בִתּוֹ שֶׁל תֶּרַח וְכָךְ הוּא אוֹמֵר לְלוֹט כִּי אֲנָשִׁים אַחִים אֲנָחְנוּ (בראשית י״ג:ח׳):
אחותי בת אבי היא SHE IS MY SISTER, THE DAUGHTER OF MY FATHER — And the daughter of the same father is permitted to be married to a descendant of Noah, for the paternal relationship is not taken into consideration in the case of the marriages between idolators (i.e. the “Sons of Noah”). He answered them in this way in order to bear out his former statement that she was his sister. If, however, you ask, “But was she not his brother’s daughter? (see chapter 11:29, and so she was granddaughter of Terah, Abraham’s father), then I reply, one’s children’s children are considered as one’s own children. Thus, too, did he say to Lot, (13:8) “For we are brethren” (although the relationship was that of uncle and nephew, as here it was that of uncle and niece).
אך לא בת אמי. הָרָן מֵאֵם אֲחֶרֶת הָיָה:
אך לא בת אמי BUT NOT THE DAUGHTER OF MY MOTHER — For Haran (her father was born of a different mother than Abraham (Bava Kamma 92a).