Halakhah for Yevamot 34:2
אשת אחיו שלא היה בעולמו היכא כתיבא אמר רב יהודה אמר רב אמר קרא (דברים כה, ה) כי ישבו אחים יחדו שהיתה להם ישיבה אחת בעולם פרט לאשת אחיו שלא היה בעולמו יחדו מיוחדים בנחלה פרט לאחיו מן האם
What, then, is meant by 'second'? Second in respect of her marriages.<span class="x" onmousemove="('comment',' The first marriage with her husband and the second with the levir. ');"><sup>5</sup></span> Where [in the Scriptures] is [the prohibition of marrying] 'the wife of his brother who was not his contemporary' written? — Rab Judah replied in the name of Rab: Scripture states, If brethren dwell together,<span class="x" onmousemove="('comment',' Deut. xxv, 5. ');"><sup>6</sup></span> i.e., dwell in the world at the same time; the wife of one's brother who was not his contemporary is consequently excluded; 'together' implies who are together in respect of inheritance,<span class="x" onmousemove="('comment',' I.e., entitled to inherit from one another. ');"><sup>7</sup></span> a maternal brother is, therefore, excluded.