This isn't hard to answer.
First off, asking /a/ is the wrong way to do things. Most people in /a/ have a twisted concept of real girls due to Anime- myself included. Real girls are not like it is in Koi Kaze (OP Pic related, of course), so that's beyond the question.
Incest in and of itself is banned for two reasons- religious and genetic. The religious basis against incest (That it's against "God"'s plan) is bullshit. The other concept, the genetic, is that incest makes fucked up children- which is entirely possible, but somewhat of a Russian roulette kind of thing.
Basically, as I've argued on these threads before-
For one, it's not legally sound to ban incest on the basis of having fucked up kids, because then we should technically ban retards/the blind/the infirm from having sex.
Second off, incest does destroy family boundaries, which is a problem. However, one has to prove that families would be permanently destroyed, etc etc.
Third off, guys (like myself) who have sisters know it's not that wonderful- Westermarck effect. It's fun to fap to in the taboo sense, but trust me, precious few of us actually even consider it in real life. If you don't have a sibling, trust me, it would be different if you did. Enjoy it as a taboo and maybe a sex game to play with some girl, the Westermarck effect kills it for most of us.