@Tatherwood:
Er... if all you care about is blood relations, than yes. A lot of us care about more than just that (and "incest" is still the word for those objections).
Even, say, the legal bits around grown-adult adoptive siblings marrying tend to be a bit tricky (perhaps not in Japan, I don't know).
Beyond any subjective yuck factor, in particular some of the non-health-related problems of incest are still a problem in this scenario (for instance, the inability to pack up your bags and leave if, say, the relationship turns sour).