[Lucarion]

OK, wild guess, just throwing this theory out here:

I have this weird feeling that Rokuro might actually be Benio's REAL twin brother. I mean, how can I not?

1) The title is "Twin Star Exorcists".
2) Yuuto doesn't look like any of the members of his family. Take a good look at that picture of the Adashino family together and replace Yuuto with Rokuro. He's a much better fit.
3) This is Japan we're talking about here and this story has ties to Japanese mythology and mysticism. The country's history and folklore has quite a number of stories and legends regarding sibling couples. Especially in regards to twins who are said to be the reincarnations of lovers. Heck, even their creation myth is about a pair of married twins. I've seen the trope in other stories before too. There's also the whole "keeping the bloodline pure and strong" thing nobles and magical families tend to do.
4) The Adashino family gives girls precedence over their male members. So Rokuro might have been given away to another family who will be able to nurture his potential. Or maybe there was some kind of prophecy or plan and they sent him away to fulfill it or protect him.
5) Why give Benio a TWIN brother in the first place? If Yuuto was a year older or a year younger, it would still have served the purpose of the story. But why give her a TWIN? I believe its because her having a twin is going to be a significant thing later in the story.
6) Apart from both being very strong exorcists, they also seem have the same weird mentality in regards to a few things.