I just go a step further - being that no other relation seems to make sense. Her Force visions are Luke's memories, she dreams of where Luke is (see: Ren's interrogation of her), and everything about her journey from the start is an homage/play on Luke.
She gets the visions when
picking up Luke's lightsaber.
The visions can easily be interpreted simply as the path that led the lightsaber to her - from Bespin (where it was first lost), to the sacking of Luke's academy (which is likely what led to Rey being left on Jakku based on the chronology of the vision), to Rey being left on Jakku to get swept up by the events of the film.
I think it's rather telling that it starts with Bespin and
not with Obi-Wan's hut, the place where the lightsaber was first passed to Luke.
I can't think of anything else she could be ... his sister? No.
Yeah, she's clearly from a successor generation.
His niece/Ren's sister? Possible, but highly unlikely (requires more gymnastics).
Not... really? Because Han/Leia never acknowledge the relationship during the film? Wouldn't that be equally problematic if she's their niece? Is there some other hurdle that I'm not aware of?
The movie goes out of its way to intentionally hide any conversations either of them could have even possibly had about her origins, with the scene in Maz's pub cutting away from Han when she asks him who Rey is, and with Leia's first mention of Rey referencing a conversation between her and Han that
also happened off-screen.
So there's
some significance to Rey that's at least known to Han and probably to Leia, but we don't know what it is, and it seems pretty obvious that they don't intend to let anything on to Rey. It's very probably a familial relationship, but that doesn't necessarily push toward any particular relationship - daughter or niece - if in either case they're compelled for whatever reason to hide this from her.
Some people point to the suggestions that Rey is inheriting the lightsaber from Luke as the reason why they're sure she must be his daughter. But so far that's the only concrete thing that makes a Luke relationship particularly likely vs. a Leia one (and, indeed, Rey could be inheriting the lightsaber through her connection to Anakin/Luke on Leia's side). Inheriting Force sensitivity obviously doesn't favor either Luke or Leia.
Rey actually inherits the most from Han: the blaster he gives her (with a warning that she has a lot to learn, which positions her as following in his footsteps), his mechanical knowledge of the Falcon (the film draws
direct parallels between her and Han in this department), as well as the Falcon and Chewie by the end of the film.
Suffice to say I think the film hasn't closed off either of the major possibilities for a familial relationship (either through Luke or through Leia/Han) yet. And I think that no matter what the final outcome is, the film is deliberately trying to suggest bits of both and in the end one of those options will have been misdirection.
A family friend? Wouldn't explain her overpowering Ren for a piece of the Skywalker lineage (with strong Sword in the Stone symbolism), nor the strong ties to Luke.
I don't think this means anything other than that she's stronger with the Force than Kylo, and in the context of the duel - with Kylo being physically and emotionally scarred and Rey finally
overcoming her own baggage - it's sufficient to examine that moment by looking at the two characters' development.