It seems she is his cousin, so "incest" tag missing?
"...I spent the summer at my cousin's place in the city." - except this line, there is no mentions in story who she might be to him, so I assume she is his cousin.
Ah, sweet, sweet celebratory Vanilla loving.
Great ending though. Heh, her face when he said that was just priceless.
Well, I don't see a reason she has to apologise. I mean, he even admitted that he felt good, basically that he enjoyed it.
But hey, at this rate, she can keep giving him 'advice' while he's there. One thing I was expecting/hoping for was that while she was bugging him about that other girl, he'd say something like he'd rather be with her instead? Didn't happen unfortunately, but it's nothing a sequel can't fix! So, y'know, it'd be nice if we get one.
Opinions may vary as to whether the 'Vanilla" tag applies properly to this, as male protag-kun seems to have this Asada-san girl in whom he's IN-terested, even if they don't seem to have gotten far, ... yet. Also, unless per chance there's a conspiracy afoot between Chihiro and his parents, THIS was probably not the sort of "cramming" they wanted him to engage in while away from home.