Incest is taboo in practically every successful human society and with good reason. It quickly leads to genetic degeneration. There are plenty reasons for that. Inbreeding leads to homozygocity. To explain that every gene has multiple different variant and we all have two copies of each gene. Inbred individuals tend to have the same variant for both copies of each gene, while outbred individuals tend to have different copies. Practically every human has some gene variants which are defective in some way, but they are not visible because only one of the two copies is a defective variant. Since inbreeding tends to produce ppl who have the same variant for both copies, they end up having two copies of the defective genes and manifest disease. Even more importantly there is an effect known as co-dominance, where two different variants work together to produce a different effect from what would be produced if the person had a double copy of each variant. I.e. If the two gene variants are A and B, then a person with one A and one B, manifests a different effect from either AA or BB individuals. Co-dominance is responsible for a phenomenon known as hybrid vigour in plants: In some cases plants arising from crossing two different varieties of a species have much better properties that either of the parents (e.g. grow faster, become bigger, produce more fruit). Similar effects are probably important in humans and give individuals that tend to heterozygocity (the opposite of homozygocity) improved abilities.
As a result inbreeding quickly results in a degenerated population, with low reproductive potential and high disease incidence. So if you just saved two individuals of a particular species and killed everything else, the species is as good as extinct. The two individuals you saved would only produce a population, which will degenerate with the span of a handful of generation, due to low reproductive success and an increased incidence of individuals of low ability. Such populations quick fall prey to predators or will fail to compete with other species for resources and naturally go extinct. Most species are in constant competition with other species and survive because their abilities make them the best at competing for a particular type of resource. Inbred populations however lose that competitive edge and lose out against species that a normal outbred population can keep at bay.
Discouraging incest has absolutely nothing to do with god. Religion codifies into dogma the survival lessons that a society learns as it evolves. Religions survive because their edicts work well for large numbers of people, which is why religions often have very adherent followers and overzealous proponents. Its evolutions by trial and error that follows similar principles to darwinian evolution. Marriages between siblings are illegal and if they occur they would have no legal standing. I am not sure what the legal framework is for sexual relationships between siblings that are adults. I expect that there would be some sort of intervention. Probably ordered into counselling by a court. However, sexual relationships of any sort between parent and child are definitely illegal and treated as statutory rape/abuse by the parent. That means even if the child is perfectly willing or an adult, the parent can be convicted of a criminal offense and sent to jail and if the child is a minor they would be taken away from them by social services. This is a good and proper thing regardless of religion.
On the flip side, exactly because it is taboo, incest holds a horrifying type of fascination for many people. The temptation of the forbidden fruit makes it appear delicious, even when its actually nothing special in the end. A lot of people love to read/watch something that is a bit horrifying, because its more exciting, so incest is a common theme in literature though in the west it is typically portrayed as destructive. For cultural reasons, which I will not go into the Japanese appear to have a stronger fascination and less inhibitions when it comes to incest. For example it is common for young siblings to share a bed even if they are not of the same sex. In general the japanese have closer family relationships than we do in the west, like bathing together and having more skin to skin contact with their young children. I suspect that the japanese might have a slightly higher tolerance for incestuous behaviour than we do i.e. different notions of propriety when it comes to family relations, but I don't actually have any evidence to support that.
That being said there is no reason to suspect that Terumi's aunt is not a blood relative. The fact that she is portrayed as very attractive supports a blood relationship, since apparently the protagonist is described as very pretty. It makes perfect sense that his close family relatives would be very attractive.
Edited by Havoc12, 19 August 2012 - 05:50 AM.