16If a woman approaches any animal to mate with it, you must kill both the woman and the animal. They must surely be put to death; their blood is upon them.
17If a man marries his sister, whether the daughter of his father or of his mother, and they have sexual relations, it is a disgrace. They must be cut off in the sight of their people. He has uncovered the nakedness of his sister; he shall bear his iniquity.
— Leviticus 20:16-17

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Article by Joe Beam

So what does the Bible say about all of this? Joe Beam addresses what is Biblically appropriate and not appropriate for Christians in their sexual relationships.