Commento su Levitico 20:21

נדה הוא means, this cohabitation ought to be kept at a distance (avoided) and is loathsome. Our Rabbis explained that these words prohibit the slightest approach to her [I.e. one is liable for superficial contact, as is defined in the commentary on verse 18] just as though she were a נדה in the case of whom such intimacy is expressly stated as forbidden, as it says, (v. 18) “he hath uncovered her fountain” (cf. Yevamot 54a).

אשת אחיו, נדה היא, “the wife of his brother, is as forbidden as a menstruating woman.” Our sages (Yevamot 54) interpret this somewhat unclear wording to mean that even incomplete coitus with one’s sister-in-law is as serious a sin as sleeping with one’s own wife while she is menstruating. The penalty makes this point quite clear. Some commentators feel that the Torah was especially harsh here, seeing that under certain circumstances, such as the brother having died without ever having had children, it may actually become a mitzvah to marry this sister-in-law who had become widowed by performing the levirate marriage ceremony with her.

נדה היא, “it is equivalent to sleeping with a woman during her period.” The comparison with a woman during her menses is that just as his wife is permissible to her during certain periods and not others, so a brother’s wife if she had become a widow and had never had any children would not only be permissible to him, but he would fulfill a commandment by marrying her in a levirate ceremony.