Reference for Makkot 10:14

עד שלא יאמר יש לי מן הדין מה אם הוזהר על בת אמו שלא בת אביו ובת אביו שלא בת אמו בת אביו ובת אמו לא כל שכן הא למדת שאין מזהירין מן הדין

Without these words, I would still be able to derive it [the scriptural warning for a full sister] with a fortiori, If one gets a scriptural warning for 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], shouldn't he certainly [be scripturally warned] for the daughter of his father and the daughter of his mother [his full sister]? Thus we see that one cannot derive a scriptural warning from a fortiori.

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