יבמות ג׳Yevamot 3

Mishnah 1א׳

אַרְבָּעָה אַחִין, שְׁנַיִם מֵהֶן נְשׂוּאִים שְׁתֵּי אֲחָיוֹת, וּמֵתוּ הַנְּשׂוּאִים אֶת הָאֲחָיוֹת, הֲרֵי אֵלּוּ חוֹלְצוֹת וְלֹא מִתְיַבְּמוֹת. וְאִם קָדְמוּ וְכָנְסוּ, יוֹצִיאוּ. רַבִּי אֱלִיעֶזֶר אוֹמֵר, בֵּית שַׁמַּאי אוֹמְרִים יְקַיְּמוּ, וּבֵית הִלֵּל אוֹמְרִים יוֹצִיאוּ:

If there were four brothers, two of them married to two sisters, and they died, the surviving two perform chalitzah, but not yibum. [For since both women are linked to each brother, the first to perform yibum violates "the sister of his linked one," who is like his wife.] If they were beforehand and married them, they must send them away. R. Eliezer says in the name of Beth Shammai: They may keep them, and Beth Hillel say: They must send them away. [The gemara reverses this, viz.: Beth Shammai say: They must send them away, and Beth Hillel say: They may keep them. For in all places, Beth Shammai's is the stringent ruling, and Beth Hillel's the lenient, except for those noted instances where the opposite is the case. And in general we say: "Beth Shammai in the place of Beth Hillel is not a Mishnah." That is, wherever we find in the Mishnah that Beth Shammai's is the lenient ruling (where it is wont to be Beth Hillel's, so that Beth Shammai stands in the place of Beth Hillel), we assume it to be not a (bona fide) Mishnah, but mistaken, and we reverse it (unless it be one of the noted Mishnayoth where Beth Shammai takes the lenient view and Beth Hillel the stringent one. And in Eduyoth, it is listed in the name of R. Eliezer among the lenient rulings of Beth Shammai and the stringent ones of Beth Hillel.)]

Mishnah 2ב׳

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.