Leviticus 18:1-3, 6, 9

Then the LORD spoke to Moses, saying, “Speak to the children 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, and you shall not do what is done in the land of Canaan where I am bringing you. You shall not follow their statutes (practices, customs).

‘No one shall approach any blood relative of his to uncover nakedness (have intimate relations). I am the LORD.

You shall not uncover the nakedness of your sister, either the daughter of your father or of your mother, whether born at home or born elsewhere.