I think the two most likely possibilities are that Rey is either Han and Leia's daughter or Luke's daughter. However, there are good arguments against each one.
1. If she is Luke's daughter, this raises the question of who the mother is. For them to suddenly introduce a major female character that we have never heard of before seems like a bit of a stretch.
2. If she is Han and Leia's daughter, why did neither of them seem to recognize her? Why didn't she recognize either of them?
It is possible that her memory was somehow modified after she was left on Jakku, or that other people's memories were modified so that they would not remember/recognize her. It is also possible that her parents think that she is dead.
At the very least, I think it is quite likely that she was a student of Luke's who survived the massacre when Kylo Ren turned Dark and killed the rest of the students. She was then probably taken away for safekeeping. If I had to guess at this point, I would lean towards her being Han and Leia's daughter, just because a confrontation between brother and sister seems more dramatically intense than one between first cousins.
I think we will just have to wait until May of 2017 to learn the answer!