This might be wild, BUT, I think they're related by blood? Possibly a younger sister or cousin? Like what if Han and Leia had rey some years after kylo and when he destroyed the temple they sent her away so she'd be safe? And maybe kylo's heard about her but hasn't seen her because he was in training? There's some dialogue in tfa book that isn't in the movie when the snow fight scene happens saying, "It is you" so he's possibly connected the dots? And with the whole "ur perents r nobody, they just sold you, ur nothing too" that kylo tells her in tlj could be him lying to trying and get her over to the dark side with him so she's no longer a threat.
This is why I don't like r*ylo because if they end up related it's inc*st and that's no good