With the information at hand, I think she's a Solo. Luke knows the dangers of a Jedi having children so he wouldn't have any. He's got a bunch of trainees to look after anyway. Han and Leia have a tumultuous relationship, on and off, as attested by their reunion in the movie. He has left several times. They bore Ben, tried to keep it together and Luke started training him. Han leaves, becoming an absent father to Ben for a time. He returns, reconciles with Leia, and they bear another child, Rey (or whatever her real name is). Our new ray of hope comes into a messy family, and Snoke is poking and prodding about. Things really fall apart this time. Ben falls to the Dark Side, and he and his Knights of Ren slaughter all of Luke's apprentices, save one: his sister. Either he can't or won't kill her, or Luke rescues her, or Han or Leia or all of them rescue her, and it's decided she will be sent to a safe planet away from danger. The difference in the actors' ages is nine years, which puts Kylo in his later teens when he turns, Rey somewhere in the 6-9 age range which fits the vision in the movie. Later in Awakens Kylo is sharply interested when the mention of a girl comes up. And when they 'saber fight Kylo offers to train her instead of killing her, like the vastly superior Darth Vader offers Luke in Empire. This time, however, the better trained warrior has character flaws and is not in charge of his emotions, while his opponent is focused and secure within herself, despite her lack of training. Another messed up Star Wars family!