Negative Commandment #131
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
131. It is a negative commandment, "do not have relations with one's wife's sister during the lifetime of his wife."
As it states in Vayikra (Leviticus) 18:18, "you shall not take a woman and her sister to be rivals, to uncover her nakedness during her lifetime." The moment a woman is married, her sister is forbidden. There is no difference if she is a sister from the father or from the mother, even if it was an illicit relationship. If he has relations with his wife's sister during the lifetime of his wife, even if he has divorced his wife, he is spiritually excommunicated. If it was done in error, a sin offering is required.
This applies in all places and at all times.