I was completely expecting Luke to say that to Rey after they met, went inside, cracked open a few beers, and got to talking. Or something like that. Instead, the way it was building up, I was already guessing that they were just going to end it with her finding Luke, standing there on his island contemplating life or maybe his next independent musical project, and that's what they did. I liked how they ended it, but that did mean that the spoken line was never actually in the movie. The true definition of tease. A glimpse of something you'll never get.
Also, upon reflection, the line itself doesn't mean that she's his daughter, niece, or whatever. He's just saying she also has The Force.