I thought: Surely the fear of God is not in this place; and they will slay me for my wife's sake. And moreover she is indeed my sister, the daughter of my father, but not the daughter of my mother; and so she became my wife. And it came to pass, when God caused me to wander from my father's house, that I said unto her: This is thy kindness which thou shalt show unto me; at every place whither we shall come, say of me: He is my brother.

To Abimelech in the wife-sister narrative of Genesis 20:11 - 23 (KJV)