Rey has to be Leia and Han's daughter. Therefore making her and Kylo siblings. Wouldn't be surprised if they are twins.
My first impression is that Rey is Luke's daughter, not Han and Leia's.
If Kylo and Rey are twins, why would they banish Rey to Jakku and keep Kylo to train? I took the leaving of Rey on Jakku to be a knee jerk reaction from Luke, when Kylo went bad. The girl looks about age 4-5 that is left on Jakku in the flashback, so I took it that Luke is training Kylo, Kylo goes bad around age 14-15, Luke decides the only way to save his young daughter and everyone is to not train his daughter and that he has to go into hiding.
That is just my theory though and the more I think on it, I could see it being sold as Rey is Luke's daughter in the first film. Not out right said, but hinted at. Then in VIII the big reveal will be that Kylo and Rey are brother and sister.
Or, I am way overthinking all of this