I just read an interesting theory by a Pixar employee that Reys parents were in the Knights of Ren. Luke gave the lightsaber to her parents to be the heir knowing Kylo would turn. This angered Kylo and killed them. Similar to the end of Ep. 3, Luke hides Rey on a desert planet, away from the evil force users. I get the twist will probably be something simple, but if Rey isn't related to a Jedi, Sith, or Force user, I don't buy that her power over the lightsaber is greater than Kylo's