edit: for the record, I'm actually more fond of the "she's a nobody" idea. I think it adds much more weight to the "meaning" of her story, which is that with a little faith in what the universe has given you you can do amazing things.
But if we're gonna put out theories based on the connections Rey has to other characters, I think the more subtle inheritance cues from Han/Leia (in particular that she explicitly inherits the lightsaber as a Skywalker heir but also subtly inherits all of Han's shit when he dies without anyone mentioning it at all) make for a stronger and more plausible guess at what "twist" might actually be in store for her origins than that the "on the nose" idea that she's Luke's daughter.
Han makes no mention of leaving a daughter on Jakku and Leia makes no mention of it either even after they meet.
And neither of them mention having a niece, either.
It's rather convenient that Han's able to pick up the Falcon almost immediately after it leaves Jakku's orbit, though. And that he lost the Falcon on Jakku, the same planet where Rey is and where the dude holding the missing piece of the map to Luke is. And that the region where Rey lives is conveniently the one he didn't check when looking for the Falcon.
And, as you said, it's clear he knows who she is.
So
all theories must grapple with the question "if he knows, why not tell her?"
And considering how open they are about Kylo Ren being their child, them being hush about a second child would be out of character.
Given how afraid they were about him turning out like Vader, and how Leia was actually a secret Force-attuned daughter in the OT that Obi-Wan kept secret for two whole movies, and on top of that how everybody seemed to think it was essential to keep quiet that Luke was a descendant of Vader, there's actually a massive precedent for being hush-hush about a second child and about a potential Jedi's parentage.
On top of that, it's heavily implied that a big reason Kylo became so power-hungry and obsessed with Vader was because he knew about his parentage.
As I said, you have to assume they're being quiet about her being their niece anyway. If you can make that leap, you can make it for them keeping their child secret for her own welfare.
I don't think Maz would have worded it in a deliberate parent to child fashion if it was parent to child to niece.
I don't think Maz would have told Rey that the family that left her on Jakku "isn't coming back," and then say that "there's someone who still could" (about Luke) if her family is actually just Luke. I suppose there could be a mother figure who's died.
Of all the people in the film, the one person who fits that bill is Leia, who's off doing Resistance stuff while Luke is a recluse and Han is apparently still keeping an eye on Jakku.
