Full-Blood sister?

This is my first book, and as i see my favorite one, i love Percy Jackson the story takes place when the Mark of Athena is over and Percy is 19. Prologue:My name is Helen Jackson, Hell for short, no i am not Percy's twin sister, i am just his sister, so yes, i am a daughter of Poseidon, and i am quite an arrogant person, but it all has it's reasons even my short name... All credits to Rick Riordan!