I saw the movie a second time, and it was interesting that both Maz and Kylo implied they had some knowledge about a mysterious girl. Maz asks Han "who's the girl?" and Kylo says "what girl?," and both times the scene changes immediately, implying both of them know something we aren't supposed to know about yet. Then Kylo says something like "it's the girl I've heard so much about," again implying there's more about her than just what's happened in the movie.
If I'm guessing, I think Rey has a direct connection to the first temple of the jedi. I don't think she is blood-related to anyone directly. Luke found her somewhere and took her in to his academy, but I think she is a direct descendant of the original force users, or something like that, and maybe she doesn't even have parents at all. She may also be related to Anakin, who of course didn't have a father.
Her being a biological child of Luke or a Kenobi relative would be so lazy and boring at this point. I really hope that's not it.