I don't see how she's Kylo's sister as their parents would never abandon her, and if she was kidnapped and left there, they'd be looking and talking about her.

I really love the idea that she's someone the Force is strong in, presumably like past Jedi; they must have appeared from families who either didn't have any talent, or never realised it, as Luke may never have without tuition. I so hope they don't retcon her background, though I know Ren could well be lying to make her need him more. And the child at the end has nascent Force.

Sorry, I haven't read any books or seen anything but the films.