In a sense, Star Wars could be considered an intergalactic episode of Maury. Probably the most famous line from any Star Wars movie was Darth Vader's "Luke, I am your father," and now the biggest question on everyone's minds is who will say that to Rey. Han and Leia probably aren't Rey's parents, despite the the popular fan theory that they are, but fans have a lot of different theories about Rey's parentage that could still be confirmed in The Last Jedi. The mystery still remains, as Rey could still be Luke Skywalker's or Jyn Erso's daughter, or perhaps Obi-Wan Kenobi's granddaughter. Each of those theories still have plenty of evidence to back them up, but fans should probably rule out the Han and Leia parentage theory now.
As CinemaBlend points out, the most compelling evidence that Han Solo and Leia Organa are not Rey's parents is present in 2015's The Force Awakens. If Han and Leia had in fact left their daughter on Jakku, their reactions to meeting her later in her life would likely have been different from what they were in The Force Awakens. Neither Leia nor Han had particularly emotional responses to meeting Rey, which you wouldn't exactly expect from someone's actual parents. The theory that Han Solo and Leia Organa had a second child was largely due to the Star Wars novels that, so far, don't really have anything to do with the new Star Wars trilogy's timeline.
In the Star Wars novels, Han and Leia had twins named Jacen (potentially Ben Solo/Kylo Ren) and Jaina (potentially Rey), the former of whom turned to the Dark Side. In the books, Leia hides Jaina to keep her away from her evil brother, but in The Force Awakens, Leia does not exactly treat Rey as a daughter. Instead, she focuses on her confirmed offspring, Kylo Ren. Han does not treat Rey like a daughter either, which could mean that he did not know that Leia had twins, but even for a series that previously featured secret twins as a major plot point, that's almost completely unbelievable.
Even further proof that Han and Leia are not Rey's parents came straight from The Force Awakens' director, J.J. Abrams. According to CinemaBlend, the director told a young fan that Rey likely hasn't met her real parents yet. "Rey's parents are not in Episode 7 [The Force Awakens]," Abrams said. "I can't possibly tell you who they are at this moment. But all I will say is that this is something that Rey thinks about, too." If that doesn't convince you that Han and Leia aren't Rey's parents, probably nothing will. You might want to believe that Han and Leia gave birth to a non-demonic child, but sadly, if Star Wars proves anything it's that you can't control who goes to the Dark Side.
The Last Jedi still might reveal Rey's parentage, and it still might involve one Skywalker parent. The most compelling fan theory about Rey's parents is that Luke Skywalker is her father, which would make a lot of sense considering that Rey received his light saber in The Force Awakens. This theory will likely receive its highly-debated answer in The Last Jedi, too, because the upcoming Star Wars movie focuses mainly on the exiled jedi.
Considering that Luke Skywalker appeared in The Force Awakens, Abrams' claim that the film did not include Rey's parents might disqualify him from the who's-your-daddy running, but since his appearance isn't until the very end of the movie, it might still count. If not, the theories that Jyn Erso is Rey's mother and that Obi-Wan Kenobi or even Sith Emperor Palpatine are Rey's grandfathers could still come true.
If Rey isn't Han and Leia's daughter then she is not Kylo Ren's sister — which also means she could have a romance with him without it being creepy. That opens up a whole new slew of possibilities, and the only way for anyone to find out what will happen is to go see The Last Jedi as soon as possible. May the answers be with you.