Commento su Genesi 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.