Responsa for Yevamot 214:4

אמר רב מיאנה בזה אסורה לזה מידי דהוה אבעלת הגט בעלת הגט לאו כיון דאיתסרא לה לחד איתסרא להו לכולהו הכא נמי לא שנא

that at the time she became subject to the levirate marriage she is looked upon as his<span class="x" onmousemove="('comment',' The levir's father's. ');">¹²</span> daughter-in-law;<span class="x" onmousemove="('comment',' A status which she retains despite the mi'un. ');">¹³</span> similarly here also<span class="x" onmousemove="('comment',' Though her mi'un which annulled her marriage retrospectively exempted her from halizah. ');">¹⁴</span> [marriage of the rival is forbidden because] at the time of her subjection to the levirate marriage she is looked upon as his daughter's rival.<span class="x" onmousemove="('comment',' Her subsequent estrangement, effected by the minor's mi'un, cannot remove her known status of forbidden relative's rival. Cf. supra note 10. ');">¹⁵</span>

Teshuvot Maharam