Leviticus 18:1-3, 6, 9

And the LORD spake unto Moses, saying, Speak unto the children of Israel, and say unto them, I am the LORD your God. After the doings of the land of Egypt, wherein ye dwelt, shall ye not do: and after the doings of the land of Canaan, whither I bring you, shall ye not do: neither shall ye walk in their ordinances.

None of you shall approach to any that is near of kin to him, to uncover their nakedness: I am the LORD.

The nakedness of thy sister, the daughter of thy father, or daughter of thy mother, whether she be born at home, or born abroad, even their nakedness thou shalt not uncover.