Yevamot 26B

חמשה נמי! למיתה דתרי לא חיישינן.

then this should also be true in a case of 5 brothers, two of whom were married to two sisters and died, and their wives happened before the 3 remaining brothers for levirate marriage. The Talmud answers: We are not concerned over the possibility of the death of two brothers in such a short period of time.

אמר רבא בר רב הונא אמר רב: שלש אחיות יבמות שנפלו לפני שני אחין יבמין ⁠— זה חולץ לאחת וזה חולץ לאחת, ואמצעית צריכה חליצה משניהם.

§ Rava bar Rav Huna said that Rav said: In a case where 3 sisters who were married to 3 brothers became yevamot and happened before two remaining brothers who were their yevamin for levirate marriage, then this one brother performs ḥalitza with one of the women, and that brother performs ḥalitza with another one of the women. And the middle, i.e., third, sister requires ḥalitza with both of the brothers.

אמר ליה רבה: מדקאמרת אמצעית צריכה חליצה משניהם, קסברת יש זיקה, והויא לה חליצה פסולה ⁠— וחליצה פסולה צריך לחזור על כל האחין,

Rabba said to them: From the fact that you say that the middle one requires ḥalitza with both of the brothers, it can be deduced that you hold that the levirate bond is substantial and creates a familial relationship, and similarly this ḥalitza performed by each of the brothers with the third sister is invalid ḥalitza. Since the third woman is the sister of a woman with whom he performed ḥalitza, it is not possible to consummate the levirate marriage with her. Therefore, the ḥalitza performed with her is not entirely valid and does not release her from the levirate bond between her and the two brothers. And there is a principle that invalid ḥalitza must be repeated by all of the brothers; each of them must perform an act of ḥalitza with this woman.