Reference for Yevamot 44:19

ואימא פרט לאחותו מאנוסה ההיא לא מצית אמרת מדרבא

What is the Rabbis' reason? Observe, they would say, it is written, The nakedness of thy sister, the daughter of thy father, or the daughter of thy mother,<span class="x" onmousemove="('comment',' Lev. XVIII, 9, referring to the offspring of an intercourse, whether as a result of marriage or outrage. ');">¹⁷</span>

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