Commentary for Yevamot 13:6

If someone's brother was married to a sexually inappropriate female who had undergone levirate marriage, and he [the first man] had another wife, and afterwards the sexually inappropriate woman dies during the life of her husband, or she divorces him, or there was a young child that she objected to having that died afterwards, it is allowed for the other woman to undergo levirate marriage. But if she doesn't object to it in his life, and she objects after his death, than she does Chalitza (a way to get out of levirate marriage) instead of levirate marriage.