Yevamot 109A

ורבי אלעזר אוסר. וכן המגרש את היתומה והחזירה ⁠— מותרת ליבם. ורבי אלעזר אוסר. קטנה שהשיאה אביה, ונתגרשה ⁠— כיתומה בחיי האב. והחזירה ⁠— דברי הכל אסורה ליבם.

but R' Elazar prohibits this. Likewise, with regard to one who divorces an orphaned minor girl whose mother and brothers married her off and remarries her and subsequently dies, she is permitted to the yavam in levirate marriage, and R' Elazar prohibits it. A minor girl whose father married her off, in which case the marriage is valid by Torah law, and who was subsequently divorced while she was still a minor is like an orphan during the lifetime of her father, as he no longer has the right to marry her off, and she cannot become fully married because she is a minor. And if the husband remarries her while she is still a minor and then dies childless, everyone agrees that she is forbidden to the yavam and may not enter into levirate marriage.

גמ׳ אמר עיפה: מאי טעמא דרבי אלעזר ⁠— הואיל ועמדה עליו שעה אחת באיסורא. אמרו ליה רבנן לעיפה: אי הכי, חליצה נמי לא תיבעי!

Talmud: The Sage Eifa said: What is the reasoning of R' Elazar, who prohibits a man from entering into levirate marriage with a woman whom his brother had divorced and remarried? It is because she was forbidden to him at one time; when the first brother divorced her, she became forbidden to the second brother due to her status as the first brother’s ex-wife. A husband’s relatives are forbidden to the wife even after death or divorce. However, the mitzva of levirate marriage grants a special exemption from the prohibition against marrying one’s brother’s wife. In this case, if the first brother had died while they were still divorced, the mitzva of levirate marriage would not have applied, and she would have been forbidden to him. The rabbis said to Eifa: If that is so, she should also not require ḥalitza, since she is a forbidden relative.