Parashas Vayeira · Passuk 20:12
And also, indeed, she is my sister, the daughter of my father, but not the daughter of my mother; and she became my wife.
Torah Temimah points to Sanhedrin 58b.
TT lemma: אחותי בת אבי.
English: “My sister, the daughter of my father.” This teaches that a sister from the mother is forbidden to a child of Noach.
Derash on the verse (Bavli sugya) — not a formal named middah of the 13. The negative implication of the phrasing establishes a Noachide incest prohibition.
A maternal sister is forbidden to a Noachide; Avraham’s statement that Sarah was his father’s daughter but not his mother’s implies the maternal prohibition already existed.