The nakedness of your brother’s wife you shall not uncover… (Leviticus 18:16)
Just as bedding one’s stepmother would be an affront to one’s father and intimacy with a daughter-in-law would be an affront to one’s son, engaging in sexual congress with a brother’s wife would be an affront to one’s brother. (In English, “sister-in-law” can mean a brother’s wife and a wife’s sister. Here, we will use the term to mean a brother’s wife.)
As with other cases of incest, this law applies to any brother, full of half- on either side, born in or out of wedlock. And, as with other incestuous relationships, the ban on a sister-in-law remains in effect in perpetuity, even if the marriage should be dissolved by death or divorce. There is one exception, however: the case of yibum (“levirate marriage”), in which a man marries his childless brother’s widow. We will discuss that mitzvah, however, when we come to parshas Ki Seitzei in Deuteronomy chapter 25. (It’s Mitzvah #598, so we have a way to go!)
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Yevamos on pages 54b-55a and is codified in the Shulchan Aruch in Even Ha’Ezer 15. This prohibition is #344 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #126 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.