20:12כ׳:י״ב
1 א

וגם אמנה, כמו באמת וכן אמנה אנכי חטאתי (יהושע ז' כ') וכן ענינו עם המ"ם אמנם; אמר אע"פ שאני אמרתי הדבר מפחד לאנשי המקום לא כזבתי בזה כי אחוה יש לי עמה כי בת אחי היא, ובת אחי היא בת אבי כי בני בנים הרי הן כבנים,

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.

2 ב

אך לא בת אמי, כי הרן היה לתרח מאשה אחרת, ואם היתה בת אמו גם כן בזה הדרך לא היה לוקח אותה, ורז"ל למדו מכאן (סנהדרין נ"ח) כי בני נח אין להם אחוה מצד האב כי אם מצד האם, ואברהם היה יכול לקחת אחותו בת אביו ממש ובלבד שלא תהיה בת אמו, כי בת אם היא הודאית:

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.