This episode basically resolves everything set-up in the previous episode. Although people thought Kyousuke was safe having not run into Ayase during his fake-date, evidence has a way to surface in the least-convenient ways, and so Ayase took him in for "questioning". All this basically confirmed what everyone already thought -- that it was unlikely that Kirino had a boyfriend. But then, right at the least convenient moment (or most convenient for her), Kirino announced that she did in fact have a boyfriend.
The remainder of the episode is essentially Kyousuke trying to work out his feelings, with the result being that (basically) he doesn't want any guy to go out with Kirino, and that any guy that wants to go out with her has to go through him first, and prove that he cherishes her more than Kyousuke does (which, by his estimation, is a pretty damn high bar to set). Suffice it to say, the act crumbles as Kirino admits that it was all an act to get back at him, and confesses (without saying) that she feels the same way about him. And the two siblings are closer than ever.
But then, as this show is wont to do, they end the episode on another cliffhanger: