Leviticus 20:17

And if a man lie with his sister, his father's daughter, or the daughter of his mother, and he dishonour her nakedness, it is depravity: for I showed mercy with the first ones, on behalf of the peopling of the world by them, while as yet I had not promulged the law in the world: but after the law hath been declared in the world, every one who committeth these things shall be destroyed by mortality, and the children of their people shall witness their punishment; for he who hath dishonoured the nakedness of his sister shall be guilty of death.

If a man marries his sister Such a marriage must have been performed by people unaware of their relationship, which subsequently became known.

and he saw her nakedness that is, “ or he sees …” (as in, “he cursed his father, or his mother” [:9]). In this context, the word saw is synonymous with “uncovered”.

and she sees his nakedness thus the act occurred by mutual consent.

a shameful perversion [Hebrew: ḥesed] from “lest the hearer put you to shame” [Proverbs 25:10]. The word denotes an egregious sexual crime.

He uncovered his sister’s nakedness If he forced her,

then he alone will bear his sin . Scripture chooses to make the above points in the case of a brother and a sister because they grow up together, they play together when young, and frequently they are left alone together. Conversely, Scripture does not mention the punishment for sleeping with one’s son’s daughter or one’s daughter’s daughter, relegating it instead to the Oral Law, because a man is normally advanced in years and empty of desire by the time he has a mature granddaughter. For the same reason, Scripture does not mention the case of a father’s mother or a mother’s mother — because they are old.

AND IF A MAN ‘YIKACH’ (SHALL TAKE) HIS SISTER. Scripture mentions the term kichah [“taking,” which generally denotes the taking of a woman as a wife] in the case of a sister, although betrothal to her is not valid, because a brother and sister sleep together in one house, and when his desire overpowers him, he takes her and draws her to himself, and he does not have to come to her as one does to a woman who is a harlot. Similarly it is the way of Scripture to use the term “taking” in the case of all those with whom one remains alone, for a wife and her mother, [a wife] and her son’s daughter and her daughter’s daughter, a wife and her sister,, Verse 18. and a brother’s wife are all [found] with him in one house [hence Scripture uses the term “taking” in each of these cases]. For a similar reason Scripture uses this expression in the following verse, A man shall not ‘take’ his father’s wife. The expression and he see her nakedness is an euphemism, since Scripture modifies its expression in speaking of forbidden relationships. Sometimes it calls sexual intercourse “uncovering of nakedness,” as it says in the case of most of the forbidden women, thou shalt not uncover her nakedness, for it is the way of those who commit fornication to uncover her skirts, similar to that which is said, and I will uncover thy skirts upon thy face; and at times it calls it “entering,” thus: If any man take a wife, and go in unto her, and hate her; to come in unto us; and he came in unto her, and she conceived by him., 38:18. Many times Scripture calls it “lying,” and here it refers to it as “seeing,” since brother and sister lodge together and there is no need for uncovering of skirts. Similarly Scripture uses the euphemism “knowing,” as in the following expressions: And the man knew Eve his wife;, 4:1. and he knew her again no more;, 38:26. a virgin, neither had any man known her., 24:16. And Scripture states [here in the verse before us] and she see his nakedness, meaning to say that she too desired his nakedness in her heart and consented thereto. Now it mentions these expressions only in the case of a sister, because in all forbidden relations when a man approaches a woman for the purpose of uncovering her nakedness, it is generally with her consent, and otherwise she can remove herself from him and cry for help, but in the case of a brother [and sister] who lodge together, it may be that it was done without her knowledge, and therefore Scripture mentioned that she too “saw his nakedness.”

חסד הוא, the word חסד here is to be understood in the same sense as the same word in Proverbs 25,10 פן יחסדך שומע, “lest the one who hears it puts you to shame.” We also find the word having such a meaning in Proverbs 14,34 וחסד לאומים חטאת, “even the kindness of nations is a form of sin.” This is also the way Rav Yoseph translates this word in his commentary on Proverbs.