What did Yuji do to his sister Kazuki? In Grisaia no Rakuen (Paradise), after saving Yuji and retrieving the real body of Thanatos (Kazuki actually), there was a scene (aboard the ship Mihama Academy) when Yuji got pissed or irritated due to Kazuki's "bossy" attitude. Yuji yelled "Kazuki!" and then he dragged her to some place on the deck. Afterwards, when they were "finished" Kazuki said: "Dont ever do that to other girls anymore..."

My question is: What did Yuuji do to Kazuki? Is there a manga that explains this?
Really got not much reference here but there are two versions to this story in the game, the safe mode where it was Yuji giving Kazuki a shoulder massage and the 18+ mode where they had sex, not sure which side the anime adapted but I think it is the 18+ side with that ambiguous scene but it is totally up to the watcher's imagination here. In that link, the commenters mentioned what they thought of the episode compared to the game so just go ahead and read those, if that is insufficient as evidence then I will leave the proper answering to someone else. But yes, they totally did say that they had sex in the game.