Hear me out...there a some really strong theorys out there about Rey being Luke's daughter, something that would sit fine with me, but I believe there is more to support she is Han and Leia's daughter. J.J., listen up...1.) The brother/sister theme is just too strong in the original trilogy to abandon when Episode VII already is so loyal to them. 2) Kylo "sees" in Rey's mind that she sees Han as a father figure ( in such a short amount of time??) 3. The embrace that Leia and Rey share near the end is waaaay to emotional for two people who never met before and is very maternal. 4.) If she is Leia's daughter she still has Luke's blood running through her and is a Skywalker in a sense. Ok, to answer a few questions...I believe that Kylo was possibly sent to the academy with Luke before Rey possibly not knowing he had a sister. When he turned and started slaughtering everyone Luke hid Rey. Luke then told Han and Leia that Rey was killed with the other students to protect Rey from Kylo, therfore Han and Leia would never believe their daughter was out there. Luke went away and probably had a vision that when Rey returned to him it would be when he was needed most, there is quite a bit said but unsaid in their exchange at the end of Force Awakens.
That's my theory folks, hope it got you thinking.