Children of Poseidon

What if Percy wasn't the only Demigod child of Poseidon? what if he had a younger half sister that not even his father knew about due to a one stand drunken stand? What if she was the Girl who Lived on the run from England? how would that change The story as we know it? (Based on the movie universe!) Percy is 15 and FemHarry is 14! takes place after fourth year!