Or HaChaim on Genesis 12:11/אור החיים על בראשית י״ב:י״א

1 ויהי כאשר הקריב לבא מצרימה, It happened as he came closer to Egypt, etc. Our sages (Baba Batra 16) use this verse to demonstrate how chaste Abraham was, that he had not even seen Sarah's beauty up until now. We need to understand why Abraham spoke to Sarah about her beauty.

2If it was merely because he planned to have her describe him as her brother instead of as her husband, this would not diminish the danger they found themselves in. Sarah would be in danger of being killed if she resisted rape, or else would have to submit voluntarily to someone who fancied her. Abraham wanted to forestall Sarah's argument that he should never have brought her to Egypt in the first place and have endangered them both. While it is true that righteous people have to place their trust in G'd, this rule applies when one does not deliberately place oneself in a dangerous situation. No one has the right to make his survival depend on a miracle. We know this from the prophet Samuel who took along an animal to sacrifice when he set out to anoint David as king. He did this even though G'd Himself told him to go on that mission. When he remonstrated with G'd that the mission could be fatal for him if Saul would hear about it, G'd Himself told Samuel to camouflage it (Samuel I 16,1-5). Abraham therefore told Sarah that had he but known earlier how beautiful she was he would indeed not have undertaken such a dangerous journey. He would have taken her elsewhere. Now that they had already come close to Egypt he had found out more about the Egyptians' lack of sexual mores and the fact that they had ugly faces (or were black-skinned). He therefore had reason to fear for his life if he were to be known as Sarah's husband. He examined Sarah's beauty critically in order to understand the degree of danger that this could pose for him. Abraham did not describe Sarah as merely יפת מראה; he described her as אשה יפת מראה. There is a subtle difference here. He realised that her beauty was unique. Her beauty was bound to arouse the carnal instincts of the Egyptians.

3We must not question how Abraham could take a chance and take Sarah to the land of the Philistines after his experience in Egypt. Whereas the Egyptians had ugly faces, this did not apply to the Philistines. Sarah's beauty therefore did not represent such a contrast to what the Philistines were used to. This is why Abraham never ordered Sarah herself to say that he was her brother. He contented himself with describing Sarah as his sister (20,2).