Reference for Yevamot 3:2
הָיְתָה אַחַת מֵהֶן אֲסוּרָה עַל הָאֶחָד אִסּוּר עֶרְוָה, אָסוּר בָּהּ וּמֻתָּר בַּאֲחוֹתָהּ, וְהַשֵּׁנִי אָסוּר בִּשְׁתֵּיהֶן אִסּוּר מִצְוָה וְאִסּוּר קְדֻשָּׁה, חוֹלֶצֶת וְלֹא מִתְיַבֶּמֶת:
If one of them were forbidden to one of the brothers as an issur ervah [e.g., his mother-in-law or her mother], he is forbidden to take her and permitted to take her sister [for she is not "the sister of his linked one," an ervah not falling before him for yibum], and the second brother is forbidden to take both. If (one of them were forbidden as an) issur mitzvah or an issur kedushah [who, by Torah ordinance, falls before him for yibum, for which reason her sister is forbidden to him as "the sister of his linked one"], they receive chalitzah and are not taken in yibum.