Reading the raws it isn't even clear they become a couple. It's frustrating because the author has made it clear (at least from the older bro) that romantic feelings are involved (don't try to argue that it's simply brotherly love, let's be real here), yet doesn't make it official. If it's too taboo of an issue then why bother writing this story? You can't blame the readers for reading too much into hints of incest, since it's clear the author IS entertaining the idea throughout the story.
I know Right? I'm not even into incest (Miyuki from Mahouka Koukou no Rettousei is creepy and turned me off from incest romance). But if you going to hint at it you better follow through, otherwise you're just doing a bait-and-switch on the fans.
This manga its 'Ongoing' its not over yet and we dont know if they are full brothers or brothers at all, because there's something weird with michirus nature, so maybe its some kind of development in future chapthers... this is the only shonen-ai of sensei so i dont know how far she goes with the 'shonen-ai' part, so lets wait and see.

Way ist this manga tagt as shouen ai ? Shiou and Mitchirou are only brothers or ist there something more ?