Leviticus 18:1-3, 6, 9

And Jehovah spake unto Moses, saying, Speak unto the children of Israel, and say unto them, I am Jehovah 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 statutes.

None of you shall approach to any that are near of kin to him, to uncover their nakedness: I am Jehovah.

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