wow i never heard of it that way.... i always thought if you bang your blood related family members (even half blood) they'd have 12 toes and 2.5 eyes.
I trust you're trolling... At any rate, it's important to distinguish between
incest (which is a legal / social term) and
inbreeding which is the biological term. There is a lot of overlap, but incest can mean a number of different things depending on which society you're part of (for example, in some societies it'd considered incest to have sexual intercourse within a clan, even if you're not technically blood relatives). In Sweden, incest used to be known as
blodskam (~bloodshame) and included sexual intercourse with sisters - so much for twins. It is a known fact that inbred offspring has a higher prevalence of negative traits (and are more sickly), but it's not a certain thing; in fact, most breeders rely on inbreeding to enhance certain traits (dogs, horses etc). This, in turn, relies on culling the unfit offspring, which could be a problem with humans... But I digress.
I found Koi Kaze to be a solid drama; the difference in age accentuates the problems that the two face and adds another dimension to the scenario.