There might not be any legitimate or widely - accepted 'Percy x his sister love fanfiction' because it's an inappropriate and abnormal concept. The world of fanfiction typically focuses on more positive and non - controversial relationship dynamics, like friendship or appropriate romantic pairings within the bounds of what's considered normal in society.
I would highly discourage the search for such fanfiction. Fanfiction should generally be based on positive, ethical relationships. Incestuous themes can be very off - putting and can also violate the intended spirit of the original work's characters and relationships.