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Negative Commandment #127

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

127. It is a negative commandment "do not have relations with one's sister, no difference if she is from his father only or his mother only, even if it was from an immoral relationship."

As it states in Vayikra (Leviticus) 18:9, "do not uncover the nakedness of your sister, whether she is the daughter of your father or your mother, born at home or born outside." If the act was done with intent, they deserve spiritual excommunication. If it was done in error, a sin offering must be brought. If she is also the daughter of his father's wife, he is required to bring two sin offerings (one for being his sister and the other for being the daughter of his father's wife).

This applies in all places and at all times.