2,031. A Mother Who is Also One's Father’s Wife

Hilchos Issurei Biah 2:2

One who is intimate with his mother who is also his father’s wife is liable for two offerings, one because of his mother and one because of his father’s wife. This is the case during his father’s lifetime and even after his father dies. The wives of a man’s paternal brothers and those of his maternal brothers are both considered prohibited relationships; this is true regardless of whether they were born in or out of wedlock. The wife of one’s father’s maternal brother, however, is only prohibited as a secondary relationship, as has already been discussed (Hilchos Ishus 1:5). A man’s paternal sister and his maternal sister are both considered prohibited relationships to him. This is true regardless of whether they were born in or out of wedlock, i.e., if one’s mother or father acted promiscuously and conceived his sister as per Leviticus 18:9, “one born at home or one born outside.”

Hilchos Issurei Biah 2:3

The daughter of one’s father’s wife who is a paternal sister is a prohibited relationship as per Leviticus 18:11, “the nakedness of your father’s wife’s daughter, your father’s offspring.” However, if a man’s father marries a woman who has a daughter from a different man, that daughter is permitted because she isn’t “your father’s offspring” to him. Since one is already liable for intimacy with her as a sister, the Torah only mentions “your father’s wife’s daughter” in order to render one liable for this prohibition as well.