Leviticus 18:1-3, 6, 9

Then the LORD spoke to Moses, saying, “Speak to the sons of Israel and say to them, ‘I am the LORD your God. You shall not do what is done in the land of Egypt where you lived, nor are you to do what is done in the land of Canaan where I am bringing you; you shall not walk in their statutes.

‘None of you shall approach any blood relative of his to uncover nakedness; I am the LORD.

The nakedness of your sister, either your father’s daughter or your mother’s daughter, whether born at home or born outside, their nakedness you shall not uncover.