Leviticus 18:6

The first of these prohibitions is against all improper and incestuous marriages, a thing very common among the Canaanitish nations and in Egypt, even to the last degree of unnatural mixtures. Diodorus Siculus relates, that it was permitted by law in the latter country, contrary to the custom of other nations, that a man might marry his own sister. None of you shall approach The prohibition is absolute, and no advances were to be made toward its violation. Indeed the only way to avoid actual transgressions, is to resist and guard against the first motions of evil. Principiis obsta, withstand the first approach of sin, is a most important precept. And it is to be well observed, that as these laws forbade marriages between near relations, they certainly much more prohibited unchastity between them, and every approach to it. Any that is near akin to him Hebrew, The remainder of his flesh; that is, his immediate relations, so near akin to him, that they are, as we say, his own flesh and blood; such as a man's sister, mother, daughter. Indeed, had near relations been allowed to marry each other, the most mischievous and fatal consequences must have resulted from it. For being much together in youth, temptations to unchastity would frequently have been too powerful to have been resisted. But, by such a restriction as this, being taught to look upon all such intercourse as prohibited and incestuous, they were assisted to withstand temptations to evil.