It comes back to the old issue of what sex is for. In the olden days when we laid down most of our morality, sex was (ideally) only to be practiced with your spouse for the purposes of procreation; this is the same reason modern fundamentalist douchebags are so homophobic (apart from the fact that homosexuality just squicks them out). Of course we all know about affairs and prostitution and premarital sex and all that, but that was all considered "immoral", just like incest. Most of the old taboos about non-procreative, outside-of-marriage sex have kind of evaporated, but the taboo against incest has remained because it received reinforcement from the scientific discovery of the potenitally harmful consequences of interbreeding, once again missing the point that sex doesn't have to result in children (and in today's society, more often doesn't than does).
The way I consider it, what do people have sex for these days if not purely for reproduction? Usually it's nothing more than a shared act of pleasure with someone you love, or at least like, or at the very least, don't hate. If you love members of your family, there's really no reason I can think of that you SHOULDN'T share sexual pleasures with them as long as no children result. People can argue "it's immoral", but if you ask them "why?" the best they will be able to come up with is usually "it's just wrong". Hell, I don't think they can even cite the Bible- after all, in the Old Testament, Lot was raped by his own daughters and Yahveh didn't smite their incestuous asses for it even AFTER they became pregnant for it and I'm pretty sure that nowhere in the book does it ever say "don't sleep with family members" (and believe me, that book says a LOT of weird shit). If they say "it leads to inbreeding and deformed children" then you can ask "but what if it doesn't result in any children?" and they're stumped again.
So really, sheer concentrated thought leads me to the conclusion that, as long as it's consensual and handled responsibly, there's really nothing wrong with incest. I'd suggest that closely related couples who actually want to MARRY might need to be sterilised (maybe just the man so they can still have a child by non-incestuous proxy, similar to how a lesbian couple might want to), but otherwise it's cool.