Yevamot 2B

ואשת אחיו שלא היה בעולמו, וכלתו. הרי אלו פוטרות צרותיהן וצרות צרותיהן מן החליצה ומן הייבום עד סוף העולם.

And the same applies to the wife of a brother with whom he did not coexist, i.e., the wife of a man who died before his brother was born. As will be explained, the obligation of levirate marriage does not apply to the yavam in this case. Since levirate marriage does not apply to him, the yevama remains forbidden to him as his brother’s wife. And the last case is if one’s yevama had previously been his daughter-in-law, and after his son had died one’s brother married her, before he too passed away. These 15 women exempt their rival wives and the rival wives of their rival wives from ḥalitza and levirate marriage forever.

וכולן אם מתו או מיאנו,

§ And with regard to all of these women listed as prohibited sex, these halakhot apply only if they were married to the deceased brother until the time of his death. However, this is not the case if they died during the deceased brother’s lifetime, or if they refused their husbands when they were minors. This refusal is referring to the decree of the rabbis that a girl under the age of 12 whose father is no longer alive may be married off by her mother or brothers. However, this marriage is not final, as she can terminate it by performing an act of refusal, i.e., by declaring, while still a minor, that she does not desire this marriage. In this case, the marriage is annulled retroactively and she is considered as though she were never married at all.