She is a clone derived from Luke's severed (and recovered) hand.

Mark my words: Rey is a female clone of Luke using Luke’s recovered hand. Who cloned it is anyone’s guess. That is why the saber calls to her, she is of the same stuff as Luke.

**SPOILERS**

I think because she is a female, people are misdirected that she is actually a clone created from Luke’s severed hand. Related yes, but in a different way. That’s why the saber responds to her.