Yevamos 3:1

Let’s say that there are four brothers, two of whom marry sisters and then subsequently die. The remaining two brothers perform chalitzah rather than yibum. If they married the women, they must divorce them. Rabbi Eliezer reported that Beis Shammai permitted them to remain married and Beis Hillel required them to divorce.

Yevamos 3:2

If one of the sisters was forbidden to marry one of the brothers as a Biblically-prohibited relationship, they may not marry but he may marry her sister, while his brother may not marry either woman. If one of the sisters is prohibited to one of the brothers as a “mitzvah” prohibition or a “sanctity” prohibition (refer back to mishna 2:4), she performs chalitzah rather than yibum.