Related על יבמות 28:16

Four brothers, two of whom are married to two sisters, and those who married the sisters died: Behold, they (i.e., the sisters) are subject to chaltiza but not to yibbum, and if [the two surviving brothers] preemptively married them, they are taken out [of the marriages]. Rabbi Eliezer says, Beit Shammai say, they may remain, and Beit Hillel say, they are taken out. Rabbi Shimon says, they may remain. Abba Shaul says, this matter was [actually] a leniency of Beit Hillel (i.e., the opinions are reversed). If one of [the women] was the mother-in-law of one of [the surviving brothers], he is forbidden to her, and permitted to her sister, and the second [brother] is forbidden two both of them. It comes out that the forbidden is permitted and the permitted is forbidden. If one of [the women] was the mother-in-law of one of [the surviving brothers], and the other one was the mother-in-law of the second one, this one is forbidden to his mother-in-law, and permitted to his brother's mother-in-law of his brother, and that one is forbidden to his mother-in-law and permitted to his brother's mother-in-law. That is what is meant by the saying, "what is forbidden to this one is permitted to that one, and what is forbidden to that one is permitted to this one." [If] one of them was the mother-in-law of both of them, this one is forbidden to her and permitted to her sister, and that one is forbidden to her and permitted to her sister. [If] both of them were mothers-in-law to one of the them, the second [brother] is permitted to both of them. This is the general rule: Any woman [who would be] fit for him [as a wife] is forbidden to him [as a yevama], on account of (Lev. 18:18), "A woman and her sister [you shall not take in marriage, producing rivalry]." If they were forbidden to him [to take as a wife] on account of another prohibition against "uncovering the nakedness"^*i.e., any of the other forbidden relationships referenced in Lev. 18:5-17), he is forbidden to her and permitted to her mother-in-law^*alt., "to her sister" (MS Vienna, GR"A).