Why does the Pasuk not inform us that Sarah did as Avraham asked her, and told Pharaoh that she was Avraham's sister?
Ramban: Because she did not, in fact, carry out Avraham's instructions, It was the wicked Egyptians who took her by force without asking any questions, and it was when Avraham informed Pharaoh that Sarah was his sister that he showered him with presents to convince him to give her to him as a wife. 1
This explains why Pharaoh blamed Avraham, and not Sarah, for having tricked him.
Why was it permissible to say, "He is my brother"? This was likely to lead to adultery, a sin for which one must give up one's life rather than violate?
Gur Aryeh #1: Regarding Avraham, his role was passive, and his sin would be "putting a stumbling block " at most. Regarding Sarah as well, one is only obligated to refuse and die when being confronted with a choice of sin or death. 1 If Sarah were to be abducted, she would not be given such a choice. They would either murder Avraham, or take her forcibly despite her refusal.
Gur Aryeh #2: Even had Sarah in fact been unmarried, it would be sinful to take her by force; therefore any guilt would rest entirely on her abductors.
Gur Aryeh does not cite the answers that the Rishonim propose (see Tosfos to Kesuvos 3b) regarding Esther to Achashverosh, and other cases of coercion. It seems Gur Aryeh did not wish to apply them here, as Avraham and Sarah may themselves have had the status of Bnei Noach. Alternatively, Sarah would not be deemed passive, as she played a role by giving the mis-information. (EK)
What does it mean, "he will do good to me, due to you"?
Rashi: They will give to me gifts. 1
Gur Aryeh: Rashi knows that this is the "good" to which Avraham refers, for later (12:16), we in fact find that Avraham was given presents.
Would Avraham have put Sarah in danger only to receive presents?!
Gur Aryeh: The gifts were part of the plan to protect them from danger. Refer to 12:13:4:6.
Why is "Yitav" in the singular (he will do good to me)?
Seforno: If Sarah would tell them that she is Avraham's sister, each one would give him presents, hoping to obtain her for a wife.
Why did Avraham say, "Say 'you are my sister'"? He should have said, "Say, 'he is my brother,' or [alternatively], 'I am his sister'"!
Moshav Zekenim: He asked her to say that she is his sister, and she did not want to lie. He then said, I will say that you are my sister; and you will be silent.
Avraham fought against four kings. Why didn't he fight when they took his wife?
Moshav Zekenim #1: Here he hid Sarah in a box, and the king was entitled to take her, according to law of the kingdom. The kings took Lot through theft.
Moshav Zekenim #2: There, he had those students that he trained along with him. Here, he was alone.
QUESTIONS ON RASHI
Rashi writes: " They will give me presents." But doesn't the verse say, "He who hates gifts shall live" (Mishlei 15:27)? Avraham later refused to accept so much as a shoelace from the king of Sedom (14:23)! Why did Avraham accept gifts at this time?
Riva: The verse in Mishlei refers to gifts from a Yisrael.
Moshav Zekenim (to 12:16): Taking gifts would make them believe the story that Avraham was her brother. Also, one should not take from stingy people (Mishlei 23:6), like Melech Sedom, while the Mitzrim were not stingy.
Ha'amek Davar (to Bereishis 14:23): Pharaoh and Avimelech felt honored through Avraham's acceptance. Melech Sedom would feel that he gave to Avraham. Also, Avraham did not want to associate with Melech Sedom.
Sifsei Chachamim: In Egypt, Avraham wanted only small gifts. Or, he said so only to persuade Sarah to agree. Alternatively, now he had no money, and he wanted to pay the debts he had incurred on the journey (see Rashi to 13:3).
Gur Aryeh #1 (to Bereishis 14:23): The gifts of Pharaoh and Avimelech were a fulfillment of the Berachah of wealth that HaSh-m had promised him. Spoils of war, however, which came through Melech Sedom's loss, are not a Berachah.
Gur Aryeh #2: Avraham reasoned that members of the aristocracy would not take Sarah by force; he was concerned only that the baser members of society might murder him in order to take Sarah. His plan was that if the VIP's would each 1 give him gifts in the hope of possibly winning his "sister" 's hand in marriage, this would make Avraham a famous person as well, 2 thus protecting him from the rabble. 3
Kli Yakar (to 12:9): If Avraham did not accept gifts, they would suspect that he really did not want to give her to Pharaoh, and they would kill him. Avraham indeed did not want to benefit from the gifts, so he later gave them to Bnei ha'Pilagshim (Bereishis 25:6), i.e. the sons of Keturah (also known as Hagar, who was originally Egyptian).
Seforno derives this from the word "Yitav," in the singular; each individual was likely to give gifts to win Sarah's hand.
It was therefore necessary to accept those gifts; as the verse concludes, "... and my soul shall live because of you."
Avraham miscalculated, however, as it was the officers and the king himself who posed a danger.