Reference for Yevamot 109:6

או כלך לדרך זו חייב כאן וחייב בדודתו מה דודתו מן האב ולא מן האם אף כאן מן האב ולא מן האם

and he is also subject to penalty [for intercourse] with his sister; as [the prohibition of] his sister applies whether she is paternal or maternal, so here also<span class="x" onmousemove="('comment',' For intercourse with a brother's wife. ');">¹²</span> [the prohibition applies] whether he<span class="x" onmousemove="('comment',' The brother. ');">¹³</span> was paternal or maternal. But might it not be argued<span class="x" onmousemove="('comment',' V. supra p. 363, n. 11. ');">¹⁴</span>

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