Reference for Yevamot 36:13
מאי היא דתנן שלשה אחין נשואין שלש נשים נכריות ומת אחד מהם ועשה בה שני מאמר ומת הרי אלו חולצות ולא מתייבמות
afterwards, for in this case he found her permitted;<span class="x" onmousemove="('comment',' V. supra p. 104, on 2-4. ');"><sup>56</sup></span> where, however, the birth occurred first and the levirate marriage took place afterwards,<span class="x" onmousemove="('comment',' V. supra p. 104, n. 6. ');"><sup>57</sup></span> what [reason [could be advanced]?<span class="x" onmousemove="('comment',' For R. Simeon's permission of marriage. ');"><sup>58</sup></span> -He holds the opinion that a levirate bond exists<span class="x" onmousemove="('comment',' Between widow and living levir. ');"><sup>59</sup></span> and that such a bond is like actual marriage.<span class="x" onmousemove="('comment',' The widow is consequently regarded as the wife of the second brother from the moment the first died. When the third brother is subsequently born the widow has no longer any connection with the deceased brother and cannot any more be regarded in relation to the third, as 'the wife of his brother who was not his contemporary'. ');"><sup>60</sup></span>