Along with what SixSever noted, there is also the idea of genetic sexual attraction, which says that individuals are more likely to find someone attractive that is similar to themselves. Someone with a similar face, mental traits, etc., is often seen as trustworthy and a good choice for a mate. This idea is usually applied in the context of related people first meeting as adults, though. That's more why an individual might like the concept in a realistic sense, rather than a fetish-taboo based one.
The main reason that incest is seen as taboo, according to what is prolly the big theory tight now, is a psychological phenomenon known as the Westermarck effect. This basically says that people raised with one another will (usually) "reverse sexually imprint" and not be attracted to one another.
That said... (and I'm not advocated incest, just thinking out loud....) if you really think about it, the only reason it should be a thing is the greater prevalence of congenital defects in kids born to related parents. But.. once you get to like 2nd cousins or so, it's statistically not likely for anything bad to happen, really, anymore that with two unrelated parents, practically speaking. THAT said, the taboo is about having kids, so if the couple never reproduces..... *shrug* Again, just thinking out loud.
I only ever get on here to make puns and lecture.... >.>
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