Wow... I saw none of that coming. It's... so much worse than I expected. Mr. Fahrenheit tricked us all.
I too am very bothered by how young he was. It is unclear how much older his cousin was, but appears to be an adult? I think, that through Miura, this... problem was brought up but with the way Jun phrased his response about how a person "won't be able to control [themselves]" and the way Jun and Miura spoke about that relationship at the beach, it is not only made to be understandable but at least hints that it is a bit acceptable. But I suppose, through the character's eyes, that they are both dead and they just wish they could be happy. Yeah, I wish the author just made them the same age, but this author does not make anything too simple huh? I'm been sitting here typing out my feelings for a good 20mins now.
...I still don't get the auditorium scene in chapter 10. Still baffled by all the characters actions.
Thank you translator and your team on this manga! Can't wait for the next chapter.