Related על יבמות 37:6

How so, "the wife of his brother who never existed with him" (Yevamot 2:1)? [If there were] two brothers who existed at the same time, and one of them died without offspring, and the second one got up [to perform yibbum], but before he was able to perform ma'amar^*an act of betrothal preceding a levirate marriage, which is not required under Torah law but which was instituted by the rabbis with his yevama before a [third] brother was born to them, who never existed with [the first brother], the second [wife] either is subject to chalitza or yibbum. [If] he performed ma'amer with her, but before he was able to marry her a [third] brother was born, and afterwards he died, or if a [third] brother was born and he performed ma'amar with her, but before he was able to marry her the first [brother] died, the second [wife] is excluded from the name "wife of his brother who never existed with him," and the first [wife] is subject to chalitza but not to yibbum. Rabbi Shimon says, if one of [the wives] engages in relations or has chalitza performed on her, the rival wives are exempt. [If] he performed chalitza with one [of the deceased brother's wives] who had ma'amar performed on her, he [also] performs chalitza on the first [wife]. [If] he married her and then died, and a [third] brother was born, and afterwards he married her and died, both of them are exempt from chalitza and from yibbum, the words of Rabbi Meir. Rabbi Shimon says, since neither of them became betrothed on his account, and he came and found both of them in a state of permissibility, then should one of wives engage in relations or chalitza, she exempts her rival wife.

Three brothers, two of whom are married to two sisters, or to a woman and her daughter, or to a woman and her son's daughter, or to a woman and her daughter's daughter: Behold, they are subject to chaltiza but not to yibbum, and Rabbi Shimon exempts both of them from chalitza and from yibbum.