Negative Commandment #126
Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
Second Section – the Negative Commandments
126. It is a negative commandment "do not have relations with the wife of one's brother, no difference if one's brother is from the same father, same mother, or from an immoral relation."
As it states in Vayikra (Leviticus) 18:16, "the nakedness of your brother's wife you shall not uncover." If he has relations with her when his brother is alive and his brother has not divorced her, the transgressor and his brother's wife are given the death penalty of strangulation, as she is a married woman. If his brother divorced her or one has relations with her after his brother passes away (and the commandment of yibum (levirate marriage) does not apply), he is spiritually excommunicated if it was done with intent. If done accidentally, he is required to bring a sin offering. If she was required to have yibum but he gave her chalitza (which removes the designation of yibum and allows her to marry any man), he does not receive spiritual excommunication but does transgress the commandment in Devarim (Deuteronomy) 25:9, "he shall not build up his brother's house."
This applies in all places and at all times.