In book 3
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After an extended absence from the story, Sissy reappears as the archetypal heroine. We can fully expect that in Book Three, Sissy will play a savior-like role for the Gradgrind family and she will care for them as best as she possibly can. There is an intense contrast and reversal of fate for Sissy is the "once deserted" girl but now she is the one who towers over Louisa and cares for the young woman who described as a metaphorical shipwreck. The shipwreck image is furthered by Sissy's depiction as a lighthouse or beacon for she "shone like a beautiful light upon the darkness of the other."
Sissy's support and the trust other characters extend her allow her to take charge when everything falls apart.
She no longer feels confined by Mr. Gradgrind's way. Sissy is kind of above Louisa for the first time, so Sissy feels like she needs to help Louisa and take charge. (Sissy is given a purpose)