That's not really the point, but I guess it's subjective in the end.
My viewpoint goes something like this: I personally think the show is on the level of a masterpiece (no, the characters, art, etc aren't perfect, but it pushes the boundaries in exploring controversial topics, and really makes one rethink their more close-minded views). I don't think it's something I would ever be a 'fan' of, though... it's kind of like how one truly appreciates a great documentary... but you'd never say you're a 'fan' of it... but maybe that's just me.
From the perspective of evolutionary psychology, the show can bring up interesting points. Most of us, when presented with the 'realistic' incest that earlier anons were complaining about, feel uneasy. Why is that? Well, it isn't really a cultural reaction, as it's a cultural universal. Furthermore, when one TRULY examines the morality behind it, a romantic/sexual relationship with one's sister really has nothing worth complaining over.
It just comes down to the fact that incest = inbreeding, which leads to deficiencies and thus we have evolved psychological mechanisms that prevent partaking in such actions, or at least attempt to (humans, being sentient, can overcome their instincts).
The show obviously doesn't go into such scientific detail, but it basically highlights that love can blossom under any circumstance, and if you watch Koi Kaze with a discerning and accepting eye there really isn't anything wrong with their relationship.