Regarding the brother having romantic feelings for his sister, I doubt this is going to happen because of the so-called Westermarck effect that was used in Koi Kaze and is socially and historically proven. It states that siblings growing up with one another desensitize themselves from getting sexually interested in each other. From what I've read (and remembered), historical proof of this were found in the Israeli kibbutz of the 1950s where children who were in a close-knit community lived as siblings, and as a result didn't have any romantic interest in one another despite not being true ones.
This was what Koi Kaze utilized: the separation of genetic siblings from one another in their early childhood will probably have an effect in them being sexually interested in one another at the onset of their adulthood. I'm no psychology major, and I only know the general details, but I doubt that Hiro will be interested in Karada despite her more mature (and sizzling hot) look. But this is an anime, so anything can be expected ...
