Star Wars: The Force Awakens
WARNING SPOILERS!!!
When Rey was left on Jakku, her taker was very cruel. Although he was mean he knew that someone was coming for her. Not to take care of her to kill her. He decided not to turn her in to the First Order and keep her, but he would have to rid her of the memory of her parents. He wiped her mind from her parents' faces and only that. He told her that they would come for her one day. Which obviously was a lie. He kept Rey busy work for a while until she became independent. He was more cruel when she left.
Rey lived secluded and alone. Only coming to the town to trade and get portions from Unkar, the one who was left with her.When she met Finn and that trouble brewed, she met Han Solo. Han unknowing to Rey (until the ending scene: Ben killing Han) had a son named Ben. Ben when to be trained to be a jedi. He turned to the dark side by Snoke. Ben was determined to find Luke Skywalker, his trainer before the Resistance. Then Rey, Finn and Han came along. Ben needed to take care of the last ones who were strong in the force.
Rey got along with Han and Leia she also knows a lot about ships and parts. So my theory is that Han and Leia had a little girl a while after Ben. Ben became evil and had to hide the little girl. Jakku was distant and Ben wouldn't think of having a sister because he had never met her. Rey is the daughter of Leia and Han.