Clayton Endicott
Superstar
I don't think Han knew. I think she was conceived and born during one of their on again off again phases. You could see their bond towards the end of the movie.It wouldn't make sense for her to be Kylo's sister. I don't see Leia and Han leaving their daughter out there alone. Even if there was some mysterious reason for it, when Han and Leia met up with Rey they would have been much more tender and loving with her. I think they switched it up and Rey is Luke's daughter. Rey and Kylo are cousins. Luke left after Kylo went dark. Maybe he didn't want Snoke getting his hands on Rey (because of her abilities) like he got his hands on Kylo, so he left her by herself. Probably still had friends watching her from afar without her knowing. I also think that's why they didn't show Luke and Rey having a conversation because it would give it away. Check how Luke looked at her when he first saw her vs. how Han looked at her when he first saw her. Luke's her father.
The second bolded part is why I think their brother and sister. The reason why Luke and Leia were split up is the same reason why Kylo and Rey were split up. This time, shyt hit the fan, and one of them fell.