They cannot be brother and sister, unless Leia had some sort of traumatic brain injury that made her completely unaware of her own daughter. Not happening, and if it does, the entire franchise should be disbanded.
My guess is that it has to do with cloning. Maybe Rey is a clone of Kylo or even Luke. We know Luke lost his hand. Perhaps Kylo shed some blood or something when he wasted the Lil’ Jedi day care. It raises the question of who did it and why? And how does “It is you” fit in? Hmmm. Will have to contemplate.