Musar for Makkot 10:11
אמר לו לימדתנו רבינו שאין עונשין מן הדין דתניא (ויקרא כ, יז) איש אשר יקח [את] אחותו בת אביו או בת אמו אין לי אלא בת אביו שלא בת אמו ובת אמו שלא בת אביו בת אמו ובת אביו מנין ת"ל ערות אחותו גילה
He answered him, You taught us, our rebbi, that one cannot derive a punishment from a fortiori. We see this in a beraisa [From what the Torah says,] (Leviticus 20, 17) "If a man should take his sister, the daughter of his father, or the daughter of his mother", I can only derive [that a person is forbidden from] the daughter of his father who isn't the daughter of his mother [half-sister from the father] and the daughter of his mother who isn't the daughter of his father [half-sister from the mother]. From where can I derive [the prohibition for] the daughter of his father and the daughter of his mother [his full sister]? The Torah teaches us this with the words ערות אחותו גילה.