The Gemara quotes Rabban Gamliel: If two brothers married two sisters, one is a Gedola and the other a Ketana (who was married off by her mother or brother that the Kiddushin is only rabbinic) and the one who married the Gedolah dies, then the Ketana doesn't need to do Miyun. If she doesn't then "She" waits until she grows up and the other one can marry someone else without Chalitza, since she is now his full fledged wife's sister.
The question is who is he referring to by "She?" Rashi learns that it's the Gedolah. She needs to wait till her sister grows up to be married to the brother completely before she can remarry.
Tosfos asks: If we were referring to the Gedolah, why do we say afterwards "the other one can marry someone else without Chalitza." Are we not referring to her originally? Shouldn't we rather write "and she can marry someone else.?"
Tosfos rather explains that "She" refers to the Ketana. He must wait untill she grows up to have relations with her again. Even though he holds there is no Zika, so technically she's not the sister of Zika, so she's not forbidden to her. However, since relations with her until she becomes a Gedolah doesn't dismiss the sister, it looks bad, so the Rabanan forbade her.
