2,032. A Sister Born to One's Father’s Wife

Hilchos Issurei Biah 2:4

Pursuant to the previous halacha, a man who is intimate with a sister who was born to his father’s wife in marriage is liable for two offerings: one because of a sister and one because of the daughter of a father’s wife. However, if one’s father raped or seduced a woman, conceiving a daughter in the act, then one is only liable for intimacy with a sister. This is because the daughter of a woman who is raped by one’s father isn’t the daughter of one’s father’s wife.

Hilchos Issurei Biah 2:5

The sister of one’s mother is a prohibited relationship. This is true of both a paternal sister and a maternal sister, and regardless of whether she was born in or out of wedlock. The same is true of one’s father’s sister: whether paternal or maternal and whether she was born in or out of wedlock, she is a prohibited relationship to him as a father’s sister.