[Drakkar77]

Regarding the discussion on this page, the posts are too numerous for me to attempt any quotes, so I'll just summarize my thoughts.

I don't know that it was stated directly in the LN but if it's true that Rock was a siscon at the three years previous to the beginning of the story, then the reason for Kirino calling him a traitor and him telling her to 'grow up already' are probably one and the same.

It seems to me that Rock and Kirino must have some how got into a discussion about having feels for each of their own siblings, and agreed that they would not tell anyone about that discussion. Rock being the siscon that he was and Manami's ability to read people she's close to, he must have confessed the discussion to Manami.

After the eventual confrontation of Manami to Kirino about the latter's feelings for her older brother, Kirino would have known immediately that it was Rock that betrayed her. Thus the reason for what they said to each other when Rock and Kirino met before the meeting to help Manami and Kirino improve their relationaship in volume 11, chapter 1 at the Tamura home becomes clear.

Rock was very surprised. Seeing Rock, Kirino said in a blank tone.

"Ah, traitor."

"——"

After he recovered (which took a while), Rock timidly asked:

"…Could it be… you still do it…?"

Rock's meaning, "you still pine for your brother?"

"Hmm, it doesn't matter anyway."

Kirino… Did she just call Rock a 'traitor?'

I didn't understand the situation, but Rock muttered:

"…Isn't it better to say it out loud… Really...? Ah… really…"

Rock's meaning, "why don't you just tell him already?"

Rock sighed… and then he spoke as if he was talking to a kid.

"That… It has been a long time already. We are both in middle school now, right?"

I couldn't believe my ears… Rock was treating Kirino like a kid.

Rock told Kirino to hurry and grow up…

The point about Rock taking some information to his grave must have been this very secret.

"………"

Kirino didn't reply, her was expression blank. I sighed and asked Rock:

"What happened between you two?"

"Ah, that was! We —"

" — Try saying it."

Kirino interrupted.

"Ugh!"

Interrupted, Rock turned to Manami —

"A a a a a a a a! I can't!"

He screamed. I was even more puzzled than before.

"Hey, what is going on?"

"Sorry Kyou-chan! Please release me! I will take this secret with me to the grave!"

What could be so important that he'd take it to his grave? Until now I didn't know.

I didn't understand all this at the time but after reading this page's discussion it seems clear to me now that this must have been the reasoning behind those exchanges between Rock, Kirino, Manami and Kyousuke.

It's my firm belief now that this is how Manami knew about Kirino's true feelings for Kyousuke, and thus why Manami counseled Kyousuke to change his behavior after the whole Sakurai incident. Thus resulting in the cold war and the beginning of the story. Manami, albeit wrongly, thought that separating the siblings might be the best way to cool Kirino's feelings. I could be wrong, it's really hard to tell if Manami was planning that far ahead or was just trying to stop Kyousuke from being the reckless superhero he tended to play at the time. It could have also been for both reasons, this story is full of double reasons/meanings like that.

Edit: Maybe I'm just showing how dumb or how much or an inexperienced read I am but Hiro Kanzaki seems to have a lot of material that requires a thorough examination in order to truly understand/appreciate everything within the story. Honestly you don't see much writing like this anymore in novels. I know it's kinda cliche to say but a lot stuff nowadays is really to watered/dumbed down. It's very refreshing to see novels with this level of complexity still being created today!