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The first law that God gave forbidding incest is found in Leviticus Chapter Eighteen.
Notice there, that His concern wasn’t just the physical defects that can result from inbreeding. Rather, He was concerned about the matter of propriety and showing respect for your close relatives.
Leviticus 18:6-8 says:
‘No man may approach a close relative and uncover his/her nakedness, for I am Jehovah. Don’t uncover your father’s naked [body] or your mother’s naked [body]… she’s your mother, so don’t [undress] her! Nor should you uncover the naked [body] of your stepmother, because [that is the same as uncovering] your father’s naked [body].’
However, back in the most ancient times there were few humans on the earth, having a sexual relationship with anyone other than a close relative such as a brother, sister, or cousin wasn’t even an option, for there were only close relatives.
For example, in the case of AbraHam, IsaAc, and Jacob, marriage to anyone other than a close relative was considered unwise due to the corrupt practices of the nations that lived around them. Therefore, AbraHam married his half sister, and both IsaAc and Jacob married their first cousins.
For more information, see subheading Incest in the linked document, Christian Morality.