Alright so here's my take on Rey/Ben sister/brother/cousin/no relation...
I loved the movie, the more that I think about it, the more I like it. I can't wait to watch it again, which I'll be doing shortly. I'm currently reading the novelization, which is canon, so it does elaborate on a few scenes, can't wait to see the deleted scenes from the movie(or a directors cut).
My theory:
Spoiler!
Rey and Kylo(Ben) are brother and sister. Ben was being taught by Luke at the new jedi academy with possibly dozen of others. Lor San Tekka (possibly Kanan) was assisting Luke in the training. Snoke who I believe is going to have been a Senator in the New Republic with Imperial ties New Order, also has some capacity with training the new jedi starts telling Ben about his grandfather, starting to plant the seed in Ben's head.
Years continue, Ben now in his mid teens, see's his little sister Rey (if that is her real name) Brought to the jedi academy to be trained as well, but Ben has already made the choice to betray his uncle and the jedi with some fellow students (Knights of Ren), they kill everyone, with the exception of Luke and Rey, maybe Luke escapes with is niece or Luke is gone and Ben can't bring himself to kill Rey... either way Luke takes Rey with Lor San Tekka to Jakku to hide her before his self imposed exile. Han and Leia believe that Rey is dead, which would explain why they say nothing to Rey but feel close to her none the less, because they don't know it's their daughter. Ben(Kylo) knows his sister survived so that is why his curiosity to find out who Rey is gets the better of him.
Now I like this direction, but I hope Luke explains this all to Rey at the beginning of VIII, because the last thing I want is Kylo Ren saying "No, I am your brother".