What did Yuji do to his sister Kazuki?
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.