Incest in the Bible
"Cursed be he that lieth with his sister, the daughter of his father, or the daughter of this mother..."
Yet Abraham marries his father's daughter and remains God's favorite:
"And Abraham said, Because I thought, Surely the fear of God is not in this place; and they will slay me for my wife's sake. 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."