Ben is older than Rey, but I think they are brother and sister. Ben snuck Rey out of the academy, and dumped her on Jakku, before he killed everyone so he wouldn't have to kill his sister. Leia and Han think their daughter is dead and never look for her, but Han and Leia still had a soft spot for Rey because she reminds them of the daughter they lost. Ben overreacts to the mention of a girl on Jakku, he knew about Rey. Luke had the "oh crap!" face when he realized Rey was not dead.My dime store theory.....
Rey and Kylo are twins, but no one knows. Han was not there for the birth and Leia had her taken away to protect her, after feeling a strong force connection.
That partially explains Leia's feelings when she hugged her, when they met in TFA.
First drafts of TFA had Rey's name as Kira (Kylo), Rey (Ren)