Commentary for Genesis 20:12

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).

AND YET SHE IS MY SISTER, THE DAUGHTER OF MY FATHER. I know not the sense of this apology. Even if it were true that she was his sister and his wife, nevertheless when they wanted to take her as a wife and he told them, She is my sister,²⁷⁸Verse 2 here. in order to lead them astray, he already committed a sin towards them by bringing upon them a great sin,²⁷⁹Verse 9 here. and it no longer mattered at all whether the thing was true or false!
Perhaps it was because Abimelech said, What sawest thou, that thou hast done this thing?²⁸⁰Verse 10 here. meaning “What sin or wickedness have you seen in me that caused you to do this out of fear? I have never attempted to take women away from their husbands.” Then Abraham answered: “I did not know you, but I thought, perhaps the fear of G-d is not in this place,²⁸¹Verse 11 here. for in most places in the world there is no fear of G-d. Therefore, from the time I left my country and aimlessly wandered among the nations not knowing to which place we would come, I made this condition with her that she say thus in all places. For the matter is true, and I thought that by doing this, human life would be saved. I did not begin this practice upon entering your country for I did not see that you had committed any sin.” And yet she is my sister, the daughter of my father. This expression represents a different argument. Abraham said: “According to my custom, I said so for it is true, and I further thought that in case they will want her they will ask me if she is also my wife. Since your servants took her and they did not ask me any questions, I said, ‘The fear of G-d is also not in this place’ and I remained quiet.”
It is possible that he established this condition with her when they came to Egypt even though he said that he did it when G-d caused me to wander from my father’s house.²⁸²Verse 13 here. It may be that he again warned her there in Egypt at the time of the event, as I have explained.²⁸³Above, 12:11.
Rabbi Abraham ibn Ezra’s opinion is that all these words were to put off Abimelech.

אחותי בת אבי, grandchildren are treated just like children and Sarah was a granddaughter of Terach.

וגם אמנה, also you, the king himself, have sinned in taking the woman because I said she is my sister for you should have asked me if she was also my wife. The fact is that she is both my sister and my wife.

וגם אמנה אחותי היא, "Moreover, she is in fact my sister." Here Abraham accuses Avimelech by exonerating himself also from the accusation of having lied. He implied that he never said that Sarah was not his wife! He had not deliberately misled G'd-fearing people. He had said that Sarah was his sister which did not preclude her from also being his wife. It was up to the Philistines to ascertain that she was not his wife if they were really G'd-fearing people. Perhaps Avimelech gave Abraham generous gifts because he accepted Abraham's rebuke. He may have wanted to demonstrate to one and all that Abraham had not lied. Righteous people are wont to worry that one should not even think they had done something improper.

וגם אמנה, besides she is indeed my sister. The expression אמנה is equivalent to באמת, “in truth.” The word occurs in the same sense in Joshua 7,20 אמנה חטאתי, where Achan admits having stolen from the booty of Jericho. The word אמנם or אמנה both mean basically the same. Avraham in effect told Avimelech: “even though I did not protest the violence committed against Sarah this does not make me a liar, as she is indeed my sister, being the daughter of my brother, Legally, nieces are just as children as grandchildren are considered as legally equivalent to children.” (compare Rashi.

וגם אמנה אחותי בת אבי היא אך לא בת אמי. “and she is indeed my sister, daughter of my father, though she is not the daughter of my mother.” Nachmanides writes that he does not know the point of Avraham’s apology if indeed Sarah was his half-sister and his wife at one and the same time, and when the people from the palace came to take her to the palace he revealed that she was also his sister in order to deceive them about the relationship. He had thereby caused them to commit a sin if he had not previously revealed that Sarah was his wife. It makes no difference whether this claim was true or false, as he had caused them to sin the moment he claimed that Sarah was his sister. Perhaps Avraham felt the need to respond to Avimelech’s accusation: “what did you see so terrible in this place that you felt the need to be devious and to bring guilt upon us?” He felt the need to justify himself vis-à-vis Avimelech personally, who had steadfastly claimed that he had never taken women for himself whom he knew to be married to other husbands. Avraham’s point was that he had no prior knowledge of the moral ethical standards prevailing among the Philistines, and had feared that they did not observe the 7 universal laws given by G’d to Noach. He cited his ample experience during previous sojourns in different countries in all of which no fear of the Lord was customary, and in all of which he conducted himself according to this assumption. Hence his caution was not a reflection on Avimelech, personally. He had been prompted by the sole motive of saving his own life. In addition, he explained that passing his wife off as his sister had not even been a lie, as indeed she was his sister by his father. According to Ibn Ezra, all Avraham’s explanations served only to confound Avimelech, his argument was basically similar to Yaakov, when challenged as to his identity, claiming in the presence of Yitzchok: אנכי, עשו בכורך, “it is I, Esau is your firstborn.”

וגם אמנם, “and besides, indeed, etc.” The word אמנם is equivalent to אמונה בפי, “I spoke truthfully; she is my sister seeing that she is the daughter of my father.” Actually this was not quite so seeing Sarah was a daughter of Haran, Avraham’s brother, a son of Terach, as we know from Genesis 11,29 “the daughter of Haran, father of both Yiskah (Sarai) and Milkah.” We learn from here that grandchildren may be described as children. The words “daughter of my father” therefore meant “daughter of my father’s son.”

In order to make his words truthful, he responded in this fashion... I.e., Avraham should have said, “She is my brother’s daughter,” and then he would not have needed to say, “But not the daughter of my mother.” He said as he did to make truthful what he had said earlier: “She is my sister.” You might ask: In 12:19, [Avraham was asked a similar question.] Why did Avraham not respond to Pharaoh at all? The answer is: Pharaoh had asked him only once, so he did not respond. But Avimelech asked twice, therefore he found it necessary to respond. (Maharshal)

Sie war früher meine Schwester als mein Weib. Sie ist dem Geiste nach meine Schwester, Tochter eines früh verstorbenen Bruders, und mit mir unter einem väterlichen Einfluß erzogen, wuchs mit mir im väterlichen Hause zusammen heran, und weil sie so an Geist und Gesinnung mir schwesterlich verwandt war, ward sie meine Frau. Es war nämlich, den Weisen zufolge, Sara identisch mit Jiska, der Tochter des früh verstorbenen Haran, dessen Kinder nach 1. B. M. 11, 31 dem Hause des Großvaters Therach verblieben zu sein scheinen.

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).

אך לא בת אמי, however she is not a sister who was born to my biological mother. This is why she is permitted for me to marry, and she became my wife.

אך לא בת אמי, seeing Avraham’s brother Haran had been born to his father Terach by another wife than his mother. Furthermore, seeing that Avimelech was certainly unaware of this, it would have been quite in order for Avraham to marry Sarah even if she had been the granddaughter of Terach from the same wife as the one who had born Avraham. Our sages, taking their cue from what Avraham said here, state that a paternal sister is not of the same category as a maternal sister, only the latter being considered as full a sister. (Sanhedrin 58). According to this, Avraham would have been entitled to marry a first generation paternal sister, not only a second generation paternal sister such as Sarah. He would not have been allowed to marry a first generation maternal sister, however.