Negative Commandment #112
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
112. It is a negative commandment “do not have relations with one’s mother”
As it states in Vayikra (Leviticus) 18:7, “do not uncover your mother’s nakedness”. This is true even if one’s mother was not married to his father, but he was with her illicitly - she is still his mother. The same law applies during the lifetime of the father or after his death. If there are witnesses and a warning, the man gets the death penalty of stoning. If there are no witnesses, but the act is done with intent, he is spiritually excommunicated. If the act is done, unintentionally, he is required to bring a sin offering. If she is the wife of the son’s father, he requires two sin offerings (one because she is his mother and another because she is his father’s wife). According to the Law of the Sages, according to “shenus d’Rabbanan” it is also forbidden to have relations with his mother’s mother and the prohibition is extended going back through the generations (i.e. mother’s mother’s mother). Also forbidden is the mother of her father’s mother (but not extended to earlier generations on her side). The mother of his father is also forbidden and is extended to all earlier generations. The mother of his father’s father is forbidden but the prohibition is not extended to the earlier generations.
This applies in all places and at all times