R' Shimon holds that one Issur doesn't compile on a different Issur. Hence if Reuvain marries a woman (and she is now Ussur to Shimon because she's his brother's wife.) Then Shimon married her her sister, she doesn't have the Issur of being his wife's sister. It cannot take effect since the original Issur of being his brother's wife is upon her.
The Gemara asks: If so,if Reuvain dies, why can't Shimon do Yibum to his wife? (After all, she is only Usser for being his brother's wife just like all Yevamos.)
The Gemara answers that the Issur of being his wife's sister hovers over her waiting if the other Issur falls off it will take effect. So even if the Issur of being his brother's wife would fall off he cannot do Yibum since another Issur would take effect, therefore the original Issur remains.
Tosfos asks: if so, it comes out that the wife only has the Issur of being the wife's brother on her. Therefore we should allow Shimon to do Yibum on her Tzara since the first wife has no more Issur on her than any other wife, the Issur of a brother's wife. In order to exempt a Tzura she needs to have another Issur Ervah on her.
Tosfos answers : the only time that the Issur of a brotehr's wife doesn't exempt from Yibum is only one that would fall off to permit Yibum. Since this Issur doesn't fall of, therefore it remains an Ervah that exempts the Tzara.
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