I can confirm that brother-sister incest is totally a legitimate fetish! I love that stuff. (Two brothers, no sisters here, so I guess I fit into the "can't properly understand how delusional the entire concept is" category.) Not that I was hoping for an incest end in this particular franchise, but that's more due to the fact that Kuroneko and Kyousuke make such a cute couple than anything else.
That sounds about right. The forbidden-fruit angle is definitely a part of it as well, but I think the biggest draw of the fetish is the idea of automatically having such an close relationship with a girl. Living under the same roof, the unparalleled intimacy of spending your whole lives together, inevitably getting to see each other in states of undress that would be scandalous in any other context...it's a pretty unique situation, really. Like osananajimi taken up to eleven. If it weren't for the biologically hard-coded mental blocks that stop it from happening in real life, it would be hard to imagine such a setup not escalating into something more.
Oh, and I second the Koi Kaze recommendation. Anyone who's not fundamentally opposed to incest shouldn't miss that series. It's absolutely beautiful.
