Talmud for Yevamot 41:21

כלתו

maternal brother, and one's daughter-in-law.<span class="x" onmousemove="('comment',' This case is discussed infra. ');">⁵³</span> Ze'iri, however, adds also the wife of his mother's father. Said R. Nahman b. Isaac: Your mnemonic sign is, 'Above that of Rab'.<span class="x" onmousemove="('comment',' Ze'ri's addition to the limitations is one generation above that of Rab. While the latter stops at the second generation (that of father and mother) the former goes as far as the third (mother's father). ');">⁵⁴</span> Why does not Rab include it?<span class="x" onmousemove="('comment',' Ze'ri's addition, a mother's father's wife. ');">⁵⁵</span> — Because she<span class="x" onmousemove="('comment',' Ze'ri's addition, a mother's father's wife. ');">⁵⁵</span> might be mistaken for the wife of one's father's father.<span class="x" onmousemove="('comment',' Who is Pentateuchally forbidden. Were a limit to be set in the case of the former, a similar limit would erroneously be set to the latter. ');">⁵⁶</span> And Ze'iri? — Thither<span class="x" onmousemove="('comment',' To the family of one's father. ');">⁵⁷</span> one usually goes,<span class="x" onmousemove="('comment',' I.e., there is frequent social intercourse between the members of the family on the paternal side. ');">⁵⁸</span> but hither<span class="x" onmousemove="('comment',' One's mother's family. ');">⁵⁹</span> one does not usually go.<span class="x" onmousemove="('comment',' No mistake, therefore, could occur between a mother's father and a father's father. Hence no preventive measure is necessary. ');">⁶⁰</span> Is not the prohibition of one's daughter-in-law

Jerusalem Talmud Yevamot