(Mishnah): Reuven and Shimon (brothers) were married, respectively, to Rachel and Leah (sisters). Reuven died, and then Shimon's wife died. Reuven's widow is forever forbidden, since she was once forbidden.

(Gemara - R. Avina): This is like R. Heila said in the name of R. Yosi said in the name of R. Yochanan - the Isur of sisters who are Yevamos is unlike the Isur of sisters who are not Yevamos. (If they are not Yevamos, after one's wife dies, he may marry her sister. If they are Yevamos, like in our Mishnah, his wife's sister is forbidden even after his wife dies.)