Yevamot 10B

היתה אמו אנוסת אביו, ונשאת לאחיו מאביו, ומת ⁠— זו היא אמו שפוטרת צרתה.

If, however, his mother was a woman who was raped by his father and subsequently married his paternal brother, and the brother died, this is a case of his mother who exempts her rival wife. According to the opinion of the Rabbis this marriage is permitted, and even according to the opinion of R' Yehuda the marriage is valid despite the transgression. Consequently, both the mother and her rival wife come before the son for levirate marriage. Since the mother is a forbidden relation to him, her rival wife is exempt as well.

ואף על פי ששנו חכמים במשנתנו ״חמש עשרה״, יש לנו להוסיף שש עשרה כגון זו. אמר ליה ריש לקיש לרבי יוחנן: ללוי דאמר ״דאי״ נמי קתני ⁠— לתני: החולץ ליבמתו וחזר וקדשה, ומת בלא בנים, דמגו דאיהי אסורה ⁠— צרתה נמי אסירא!

And although A baraita states in the Mishnah: 15 women, we must add a 16th case, For example: the situation described above. Reish Lakish said to R' Yoḥanan: According to the opinion of Levi, who said that the Mishnah teaches even cases of: What if, which involve a transgression, let the Mishnah also teach the case of one who performs ḥalitza with his yevama and went back and betrothed her, and then died childless. His betrothal is a transgression, as after ḥalitza the yevama is permanently forbidden to him. As, in this case, since she is forbidden to all the brothers, due to the ḥalitza performed by the deceased yavam, her rival wife is also forbidden. This is an additional case of a woman who exempts her rival wife.

אמר ליה: לפי שאינה בצרת צרה.

R' Yoḥanan said to Reish Lakish: This example is not cited because the case does not include the possibility of a rival wife of a rival wife. All of the other cases listed in the Mishnah involve both rival wives and rival wives of rival wives. Here, however, this is impossible, as the same prohibition applies equally to all of the brothers, and therefore none of them may marry her.