We are often told that Abraham and Sarah lied to Pharaoh about being brother and sister:
Genesis 20:2, 4: "Abraham said of Sarah his wife, 'She is my sister.'... 4Now Abimelech had not come near her; and he said, 'Lord, will You slay a nation, even though blameless? 5Did he not himself say to me, "She is my sister"? And she herself said, "He is my brother."’"
However, shortly thereafter in the same chapter:
Genesis 20:11: "Abraham said [to Abimelech], 'Because I thought, surely there is no fear of God in this place, and they will kill me because of my wife. 12Besides, she actually is my sister, the daughter of my father, but not the daughter of my mother, and she became my wife; 13and it came about, when God caused me to wander from my father’s house, that I said to her, "This is the kindness which you will show to me: everywhere we go, say of me, 'He is my brother."'"
Why is it considered lying when, technically, Abraham and Sarah really were brother and sister?