Mishnayos Yevamos Perek 3 Mishnah 2

If one of the sisters was forbidden to one of the brothers due to a prohibition against forbidden relations because she was a relative of his wife or a relative on his mother’s side, then he is forbidden to marry her but permitted to marry her sister. Because she is his close relative, she is exempt from levirate marriage with him, and therefore she is not bound to him with a levirate bond. Consequently, her sister is not considered the sister of a woman with whom he has a levirate bond, and he is permitted to enter into levirate marriage with her. But the second brother, who is not a close relative of either sister, is forbidden to marry both of them. Indeed, for him each woman remains the sister of a woman with whom he has a levirate bond. If a prohibition resulting from a mitzva or a prohibition stemming from sanctity will be transgressed when one of the women marries one of the brothers, then her sister must perform ḥalitza and may not enter into levirate marriage, as she is considered the sister of a woman with whom he has a levirate bond. In this case, the sister who is forbidden to the brother due to a mitzva or due to sanctity is bound to the brother for the purpose of ḥalitza.

הָיְתָה אַחַת מֵהֶן אֲסוּרָה עַל הָאֶחָד אִסּוּר עֶרְוָה, אָסוּר בָּהּ וּמֻתָּר בַּאֲחוֹתָהּ, וְהַשֵּׁנִי אָסוּר בִּשְׁתֵּיהֶן אִסּוּר מִצְוָה וְאִסּוּר קְדֻשָּׁה, חוֹלֶצֶת וְלֹא מִתְיַבֶּמֶת: