Zblayde said: It would be, if they weren't RELATED! Seriously, do Japanese people think this is OK, or do mangakas just want to have sex with their sisters that badly?
It's not that uncommon of a fetish in the rest of the world, no more so than water sports, sadism, voyeurism, bestiality, double penetration, or other such not quite mainstream but not quite ultra rare fetishes. The Japanese merely think it's better to let the people who have the fetish to indulge in it through fiction and blow off their desires in that manner, while other countries believe the best way to deal with such thoughts is completely marginalize people who have taboo fetishes.
It's the ages old battle of video game violence in a different form; are violent video games mostly harmless, letting people blow off steam, or do they condone and encourage violence? In regards to incest, the Japanese simply view it as the former, while the US views it as the latter for the most part.