Leviticus 18:1-3, 6, 9

And the LORD spoke to Moses, saying, “Speak to the people of Israel and say to them, I am the LORD your God. You shall not do as they do in the land of Egypt, where you lived, and you shall not do as they do in the land of Canaan, to which I am bringing you. You shall not walk in their statutes.

“None of you shall approach any one of his close relatives to uncover nakedness. I am the LORD.

You shall not uncover the nakedness of your sister, your father’s daughter or your mother’s daughter, whether brought up in the family or in another home.