Rav Ashi (11a) answers that the Beraisa follows the opinion of Rebbi Shimon and refers to a different case from the one which Reish Lakish discusses: Reuven dies, and Shimon performs Chalitzah with Reuven's wife. Then, new brothers are born. Since those new brothers were not alive when Shimon performed Chalitzah, no Isur took effect upon them when Shimon did Chalitzah. Thus, after Shimon marries the woman and dies, they are not prohibited to his wife and they must perform Chalitzah.
DID YOU MEAN THEY DO NOT NEED TO PERFORM CHALITZAH. THEY WERE NOT ALIVE, OR HE MARRIES HER AFTER THEIR BIRTH?
MOSHE SALAMA, OCEAN, NJ, USA
The new brothers must perform Chalitzah because no Isur of Kares prohibits them to her: she is not prohibted to them as the Eshes Ach (she'Lo Hayah b'Olam) from Reuven, because they were born after Shimon married her. Although she is considered their Eshes Ach from Shimon because they were born while he was married to her, she is their Eshes Ach b'Makom Mitzvah and thus they must do Chalitzah with her.
Y. Shaw