Leviticus 18:17

Of a woman and her daughter, to wit, thy step-daughter, and so thy step-son's daughter, &c. It is wickedness; because they are very near to thy wife by consanguinity, as coming directly from her; and therefore they are as near to thee by affinity, which binds as much as consanguinity; the wife, who is only related by affinity, being nearer to a man than any other by consanguinity, they two being made one flesh, and therefore the same distance is to be observed in both of them.