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

113. It is a negative commandment “do not have relations with the wife of one’s father.”

As it states in Vayikra (Leviticus) 18:8, “do not uncover the nakedness of your father’s wife.” This is true even if she is not his mother. She is considered his father’s wife if there was erusin (first stage of marriage) or nesuin (second stage of marriage), during his father’s lifetime or after his father’s death, or even if she has been divorced. If there were witnesses and a warning, then the sinner receives the death penalty of stoning. If there were no witnesses: if he did this with intent, he is spiritually excommunicated. If done without intent, he is required to bring a sin offering. According to Shenius from the Sages, he is also considered to transgress this law if he has relations with the wife of his father’s father and this extends up to earlier generations. Also, the wife of his mother’s father, but this does not extend to earlier generations.

This applies in all places and at all times.