I was on the fence about how I felt about that topic, mostly because I never gave it much serious thought... but after watching this, I've come to the conclusion that if it's love between two consenting adults (more on that in a bit), then I generally don't have a problem with it, morally. Society does, of course, and it may hurt and confuse everyone around the couple, especially parents and family... but there's a large number of things involving sex and love that have the same effect, many of which are slowly becoming accepted, and rightfully so. Legality is another thing entirely, and one that I won't touch upon here... (edit: until later, apparently. lol)
HOWEVER... what Koi Kaze seems to want to convey is a tragic and "true love" story, and I have some problems with that. Koshiro, after discovering they were siblings, is initially consumed by physical and sexual desire, recognizes it as such, and struggles with it for the better part of 7 episodes or so. Then, inexplicably (in my opinion), it turns to an emotional need, and then love. He never says (audibly or through internalized thoughts) what it is about his sister that makes him feel drawn to her emotionally. It just "is", suddenly.
Nanoka, on the other hand, handles it with a beautifully gradual progression (although the part where she sleeps in his bed waiting for him to come home was a bit sudden). Unlike Koshiro, she doesn't recognize what she's feeling, and tries understand it for the better part of 7 episodes or so. But what's the motivation for her feeling of love?
As a few have stated here already, their feelings of love only make sense when viewed a certain way... which is largely as infatuation.
Koshiro is hung up on the physical aspect, rationalizes it with an emotional need out of no where, which then becomes a first for him, as he's never had what feels like a deep emotional need for anyone... as evidenced by his conversation with his ex. Basically, it feels like his first love, which any teenager can probably tell you, is only crazy infatuation... probably.
Nanoka is young. She also has idolized and wanted to be around her brother for as long as she can remember. That, coupled with their chance encounter before they knew the truth (which was wonderfully done, btw) equals crazy teenage infatuation, for both of them. That's about it.
So yeah, I don't see the "true real love" aspect, as there isn't much substance to it, unfortunately... I really wish there was. Granted, 13 Eps is kind of a short run, so I grant them some allowance for time. The cynical part of me wants to give it an 8/10, and another part of me that loved the story, atmosphere, music, characters (notably, Chidori), almost everything (meh art) wants to give it 10/10 anyway... so 9/10 as a compromise.
Also, correct me if I'm wrong, but didn't Nanoka have a birthday in the Ep 5? Pretty sure she was 15 when she moved in... so she'd be 16, almost 17 by the time they did the deed...
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(which, as cringe inducing as it was, was 100% necessary... how else could they irrefutably show they accepted their love for one another? A kiss or something could be passed off as awkwardly close siblings, or just some passing foolishness, but that... there's nothing more convincing, for them or the viewer)...
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...and 16 is the age of consent, meaning legal, in the majority of states in the US, and 13 (um, yup) is in Japan, though cities/prefectures may have ordinances locally restricting it to 18. So, 16 might have been risque, or maybe frowned upon a bit, but I don't see it being illegal or seriously morally reprehensible... probably, maybe.
As for the end, I'm glad it was open to interpretation... I can understand arguments for the "once a year date" deal and the everyday normal thing. Not sure where I stand on that, and I'm okay with that =P
Sorry this is crazy long! At least I can sleep now.
TL;DR - Their love didn't have any real substance to it, but I loved the show anyway.
