Percy Jackson's Sister

Maddie, never realized she was a demigod, until she was 15. Most get rescued at 12, but no one ever noticed Maddie in her small orphanage in Manhattan. That is, until one day, Maddie goes to Long Island beach to relax, until something mysterious comes out of the waters... (All characters except for Maddie and other stuff are owned by Uncle Rick Riordan!:D)