Reference for Yevamot 107:2

אלא כגון שנתכוון לאשתו ותקפתו יבמתו ובא עליה שניהם אנוסים דבי רבי חייא היכי דמי כגון שנתכוון לאשתו ותקפוהו עובדי כוכבים ודבקום זה בזה ובא עליה

and if the woman was his sister-in-law no <i>kinyan</i> is thereby constituted!<span class="x" onmousemove="('comment',' By the accidental contact. She does not thereby become his lawful wife. ');">⁵</span> — It is<span class="x" onmousemove="('comment',' Intercourse under compulsion is possible. ');">⁶</span> <font>when, for instance, his intention was intercourse with his wife and<span class="x" onmousemove="('comment',' While he was in the state of erection. ');">⁷</span></font> his sister-in-law seized him and he cohabited with her.</font>

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