It doesn't have to be a demigod. RR always gives out info about new characters a few weeks before the book is released. The characters are in the book and do affect the course of the plot, but that doesn't necessarily mean that they are the focus point of the novel. For instance, one of the new characters he released info about before the Son of Neptune came out was Ella. True, she was important, but she was not a demigod or a major character.

I think it would be awesome if Percy had a sister it would be fair too because thalia got a roman brother (Jason), nico got a roman sister (hazel),so I think it's only fair if Percy gets a sister somewhere in the series , and Poseidon did say in the end of the last Olympian something abought bring his other children out of hideing or something like that

He'll probably have a plan out for my personal Demigod
shut up I can dream