Spoilers for Star Wars: The Last Jedi ahead. Ever since Kylo Ren (Adam Driver) had a pretty intense reaction to the existence of Rey (Daisy Ridley) back in The Force Awakens, Star Wars fans have been speculating that the two somehow knew each other prior to the events of that movie. If you recall, at the very mention of a girl working with the rebellion, Kylo Ren shouted, "What girl?!" at a poor First Order plebeian and broke a lot of hardware. Since then, it's been kind of assumed that Kylo Ren knew who Rey was the whole movie and just kept the secret to himself. Well there are a few new reasons to think that not only is that true, but that the duo might be closer than you think — just take these Last Jedi clues that Kylo Ren and Rey are brother and sister.
A major plot point of The Last Jedi revolves around Rey and Ben Solo, aka Kylo Ren, being able to communicate with each other telepathically. It begins right after Luke has taught Rey how to tap into the Force. Once she can start to feel it an access it on her own, Ben appears in her mind's eye. The two can see each other, and speak to each other, despite being light years apart and on different worlds. The power of the Force is super strong, and Ben had already tapped into it, we know, since he was a student of Luke Skywalker's. But was Rey as well? The fact that they can communicate in such a way, even without Rey having had an official Force training, suggests that the pai might be related after all.
Then there's the fact that their strength in the Force itself is pretty on par with one another's abilities. The trailer for The Last Jedi gives away a little bit of this when Luke Skywalker says of Rey, "I've seen this kind of raw power only once before." Then, there's a scene towards the end of The Last Jedi when Rey and Ben have seemingly joined forces to fight Supreme Leader Snoke. But Ben's momentary return to the light is a short one; his heart returns to the dark again and he embraces his role as Kylo Ren. He and Rey both extend their hands out for the same lightsaber, reaching for it with all of their might, but the Jedi weapon floats between them, suspended in air and pulled between two equally powerful people. Since Rey has never had any real training in how to use the Force, and that all of this comes natural to her, this might mean that there's something more going on here. If Rey and Kylo Ren have equal power and equal abilities, the reason behind that might be that they're brother and sister.
Finally, there's the sexual tension. Now, I know what you're thinking, why would sexual tension mean that they're brother and sister?! Well think about what the original trilogy did with Luke and Leia. There was massive flirtation in A New Hope, and even a kiss in The Empire Strikes Back. The two were a part of a big love triangle before Obi Wan Kenobi revealed to Luke that they were siblings in Empire. In The Last Jedi, the sexual tension takes on the form of Rey and Ben's ability to basically be in each other's lives at any given moment. There's one instance of their telepathy when Rey "stumbles" onto Ben as he's not wearing a shirt. Ben's biceps are bulging and his chest looks glistening. Rey is visibly uncomfortable, and she humorously asks Ben to put on a shirt, which he ignores, and their tension is kind of left to stew.
There are plenty of people shipping Ben and Rey, just like there were plenty of fans shipping Luke and Leia ages ago. And putting them in those kind of situations would be the perfect way to throw fans off the scent of a later sibling reveal.
Though, to be honest, Kylo Ren and Rey turning out to be brother and sister would be kind of repetitive. The Star Wars saga has already done a secret brother-sister reveal with Luke and Leia, and considering there's a lot of repetition going on already from one trilogy to another, I personally am not rooting for them to be related. But considering that The Last Jedi doesn't provide any definitive answers in the positive or negative, we'll just have to wait a couple more years for the final installment to (hopefully) find out.