I also thought it was shounen ai when I first picked it up but it's not.
It's a pretty good series... I like the characters, there are definitely some funny parts, also some sad/more serious scenes. I do agree that the plot is quite repetitive. In every volume, I'm pretty sure, Kotarou gets into some sort of trouble, Yuuto worries, and Tatsuki acts like he doesn't care but then gets hurt helping out anyway.
Actually, though, I have to disagree with a previous reviewer who basically said Kotarou sucks for the way he treats Tatsuki. I can see why that would be said but if you look at it from Kotarou's perspective, it's not his fault. Tatsuki constantly acts very cold toward him, says mean things, pushes him away, acts like he's too dirty to touch, and then he'll come help out but make it seem like he never cared to in the first place. You can see Kotarou wondering at times what his cousin's problem is and starting to think it'll be okay but then Tatsuki acts cold again. (Meanwhile, Tatsuki definitely has his reasons which you'll see when you read the series). I thought the mangaka did a good job of showing those scenes in a way that made it understandable on both their parts every time, which is what is so tragic about their situation.
The plot does get more interesting at one point and there's more going on than meets the eye in the beginning. I have some problems with other parts of the story that would be spoilers if I said what it was or why but that's why I give it at 6/10. Even so, I continued to like the characters throughout the series and it did hold my attention. It's an interesting series and has its good points.