Reference for Yevamot 109:9

ואימא אידי ואידי באשת אח מן האב חדא דיש לה בנים בחיי בעלה וחדא דאין לה בנים בחיי בעלה אין לה בנים בחיי בעלה מדרב הונא נפקא

Deduction should be made concerning a relationship which is due to betrothal<span class="x" onmousemove="('comment',' A brother's wife. ');">¹⁶</span> from a relationship that is due to betrothal,<span class="x" onmousemove="('comment',' V. supra note 5. ');">¹⁷</span> but let no proof be adduced from a sister the prohibition of whom is natural!<span class="x" onmousemove="('comment',' It is due to vicissitudes of birth and not to any act of his. ');">¹⁸</span>

Jastrow