ANSWER: The incident you are referring to took place during a journey through the Land of Canaan taken by Abram and Sarai (Sarah). As they were traveling South from Bethel a severe famine hit the land. They decided to travel further south and live in Egypt for a while until the famine passed. Abraham, however, was concerned about his safety when he entered the land of Pharaoh.
11 When he (Abraham) was about to enter Egypt, he said to his wife Sarai, 'I know well that you are a woman beautiful in appearance; 12 and when the Egyptians see you, they will say, 'This is his wife'; then they will KILL ME, but they will let you live. 13 Say you are MY SISTER, so that it may go well with me because of you, and that my life may be spared on your account.' (Genesis 12:11-13, NRSV)
Sarah was asked to lie - or at least tell the Egyptians a half-truth - in order to protect his life. We discover the half-truth when, in Genesis 20, Abraham again uses her to save himself from a powerful ruler - in this case the king of Gerar named Abimelech. After being confronted by the king he admits that Sarah was more than his wife when he states,
11 'I thought, 'There is absolutely no fear of God in this place. They will kill me because of my wife.' (sounds familiar, doesn't it?) 12 Besides, SHE REALLY IS MY SISTER, the daughter of my father though not the daughter of my mother, and she became my wife (Genesis 20:11, HCSB).
Marriages between men and their half-sisters were not uncommon in ancient times. Although the response Sarah was coached to give (on two occasions) concerning the nature of her relationship with Abram was not a COMPLETE lie, it was nevertheless only part true and a deception (she was, after all, married to him!).
His concerns and fears that he would be killed for his wife, in spite of how he ultimately handled them, were not entirely unwarranted. When the Egyptians finally met Sarah and "saw that the woman was very beautiful" (Genesis 12:14, NASB) they took her immediately to Pharaoh's house! Abraham's life was not only spared but as Sarah's "brother" he was given donkeys, camels and even slaves (Genesis 12:16)!
Sarah was 67 YEARS OLD when it was believed her beauty could bring an end to her husband's life! Her looks, in both Biblical and secular sources, is spoken of in highly favorable terms. Tradition, according to the 1906 Jewish Encyclopedia, states that Abraham hid her in a box just before he entered Egyptian territory. When the chest was opened by officials to determine any duty owed, the Egyptians saw how absolutely gorgeous she looked. Tradition then says all the officials wanted her so bad that they were outbidding each other as to who would ultimately have her.
It was only after taking Sarah into his house, with plans of making her his wife, that Pharaoh discovered the deception (Genesis 12:17-20). He angrily confronted Abraham and then threw him and his wife out of the country!