“Abraham said of Sarah his wife, “She is my sister.” So Abimelech king of Gerar sent and took Sarah.”  Gen. 20:2

Abraham used this same ruse against Pharaoh with the same result of Sarah getting kidnapped, probably to his surprise.  And like father like son, this very same ruse is used again by Abraham’s son Isaac in Genesis 26.

Matthew Henry, in his Commentary on the Whole Bible, explains that Abraham sinned in this same way before, and was reproved for it, yet does it again.  Henry says of this condition “It is possible that a good man may, not only fall into sin, but relapse into the same sin, through the surprise and strength of temptation and the infirmity of the flesh.”

Abraham explains to Abimelech later that Sarah is indeed his half-sister, though outside of Mesopotamia, marrying such a close relative may have been outside the culturally accepted norm.