TT #720 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:07
And if the man does not desire to take his yevamah, then his yevamah shall go up to the gate to the elders and say: “My husband’s brother refuses to establish a name for his brother in Israel; he will not perform the duty of a husband’s brother for me.”
Torah Temimah points to Sifrei.
TT lemma: לֹא אָבָה.
לֹא אָבָה – הוּא, וְלֹא שֶׁלֹּא אָבָה בָּהּ הַמָּקוֹם; מִכָּאן לְחַיָּבֵי כְּרֵיתוֹת שֶׁאֵינָן עוֹלוֹת לֹא לְיִבּוּם וְלֹא לַחֲלִיצָה.
English: “He will not” — he (the man), and not that the Omnipresent has refused her; from here that those liable to karet are eligible neither for yibbum nor for chalitzah.
On the daf / in context: When the relationship itself is forbidden by a karet prohibition, the entire institution of yibbum/chalitzah does not apply.
Mi’ut excluding relationships forbidden by karet.
Women forbidden to the yavam by a karet prohibition are exempt from both yibbum and chalitzah.