Reference for Yevamot 51:23
לא מיבעיא קאמרינן לא מיבעיא תלתא דודאי בטלה מצות יבמין אבל ד' למיתה לא חיישינן קמ"ל
IF ONE OF THE SISTERS<span class="x" onmousemove="('comment',' In the case mentioned in the first paragraph of our Mishnah. ');"><sup>48</sup></span> WAS FORBIDDEN TO ONE BROTHER UNDER THE LAW OF INCEST AND THE OTHER SISTER WAS FORBIDDEN TO THE OTHER UNDER THE LAW OF INCEST, SHE WHO IS FORBIDDEN TO THE ONE IS PERMITTED TO THE OTHER AND SHE WHO IS FORBIDDEN TO THE OTHER IS PERMITTED TO THE FIRST. THIS IS THE CASE CONCERNING WHICH IT HAS BEEN SAID: WHEN HER SISTER IS HER SISTER-IN-LAW<span class="x" onmousemove="('comment',' The wife of her husband's brother. ');"><sup>52</sup></span> SHE MAY EITHER PERFORM <i>HALIZAH</i> OR BE TAKEN IN LEVIRATE MARRIAGE.<span class="x" onmousemove="('comment',' V. supra 20a. Cf. supra p. 162, n. 6. ');"><sup>53</sup></span>