Genesis 20:12
And yet indeed she is my sister; she is the daughter of my father, but not the daughter of my mother; and she became my wife. And yet indeed she is my sister; she is the daughter of my father, but not the daughter of my mother; and she became my wife.
Sarah was Abraham's paternal half-sister, sharing the same father but having a different mother, and she became his wife. To call her his sister was therefore true in that limited sense. That marriage to his half-sister involved no wrongdoing.
The statement belongs to the second of two times Abraham said Sarah his wife was his sister. Its setting is his sojourn among the Philistines and the events involving Abimelech, who knew the true God and had a higher regard for morality than other nations of Palestine. It falls within God's providential dealings with Abraham's family, whose line was chosen to carry forward the promised Seed.
He acted from unbelief and lack of trusting faith in God's protecting care, showing his faith was not yet perfect and that he did not always trust God. God used Abimelech to give Abraham a stern rebuke, and Abimelech later also served as a means of rebuking Isaac.
The chapter shows that the principles of the moral law, including respect for virtue in marriage and sexual morality, were known and judged conduct before Moses. The verse is an instance of violating the ninth commandment before the law was proclaimed at Sinai, showing the moral law was known before its formal proclamation.