Blowfish

Estelle Blofis, the sister of the great Percy Jackson, is in middle school. However, when her older brother and his long-time girlfriend, Annabeth Chase, drive her to school in Manhattan, the unthinkable happens, and Estelle must face her fears in order to save not just Percy, but the whole world...

(Btw, I sincerely apologize for the chapter lengths. I seemed to have forgotten that this was a fanfic and not a book when I began writing, but I just went with it.)