[DasDingus]

Originally Posted by Undertaker

Keep in mind that the original LN was told through Kyosuke's POV only. So there are a lot of self-reflection and inner dialogs in the books that were not shown in Anime has more or a 3rd person POV with Kyosuke's comments be kept minimum.

Spoiler for Kyosuke's acceptance:

But if you are interested, the reason was simple, Kyosuke was commenting on how Akagi must be lying and that there are no way a younger sister would ask her brother to buy Eroge for them until a bulb lights up that he was doing the exact same thing as Akagi. With that realization and not wanting Akagi to in turn ask him why he was there, he decided to just accept it and made a pact with Akagi that both of them will pretend they never seen each other there.

My confusion was over Kyouske going out to buy the eroge in the first place. The reasoning behind his acceptance of Akagi's excuse was fairly straightforward.