And it came to pass when I was come near to enter into Egypt, the Lord said unto me: Behold, Sarai, thy wife, is a very fair woman to look upon; therefore it shall come to pass, when the Egyptians shall see her, they will say She is his wife; and they will kill you, but they will save her alive; therefore see that ye do on this wise: Let her say unto the Egyptians, she is thy sister, and thy soul shall live. And it came to pass that I, Abraham, told Sarai, my wife, all that the Lord had said unto me Therefore say unto them, I pray thee, thou art my sister, that it may be well with me for thy sake, and my soul shall live because of thee. (Abraham 2:22-25)
Some have argued that this is an instance of “divine deception.” However, as we learn in Genesis 20:12, Abraham and Sarah were indeed siblings—they shared the same biological father.
Furthermore, while Genesis does not tell us that God was the ultimate source of this plan to save their lives, ancient texts discovered after the time of Joseph Smith, when recounting the Abrahamic narrative from Genesis, presents Abraham as the recipient of a prophetic dream instructing him to tell the Egyptians that Sarah was his sister, not wife.
The text discovered at Qumran, the Genesis Apocryphon (1Q20), column 19, lines 17-20, reads thusly:
Then I awoke in the night from my sleep, and I said to my wife Sarai, "I dreamt a dream (and) on acco[unt] of this dream I am afraid." She said to me, "Tell me your dream, so that I may know (about it)." So I began to tell her this dream, and I said to [her], " . . . this dream . . . . . . that they will seek to kill me, but to spare you. Therefore, this is the entire kind deed th[at you] must do for me: in all cities (?) that [we will ent]er s[a]y of me, 'He is my brother.' I will live under your protection, and my life will be spared because of you. (Daniel A. Machiela, The Dead Sea Genesis Apocryphon: A New Text and Translation with Introduction and Special Treatment of Columns 13-17 [Studies on the Texts of the Desert of Judah vol. 79; Leiden: Brill, 2009], 71-72)
For those interested in the relationship between the Book of Abraham and Abrahamic traditions found in Jewish, Christian, and Islamic texts, one should try to track down a copy of John A. Tvedtnes et al., Traditions about the Early Life of Abraham (FARMS, 2001). Sadly, the book is out of print, but Jeff Lindsay has a very good summary of the book at: