There's a law against it because it is commonly accepted that it is not acceptable. I already explained why. Consider also that the possible result of sex is always offspring, which we have determined in this case will not be healthy. Yes, you can use protection, but what if it fails? What if she forgot to take the pill? These are possibilities that can't be denied. And then what? Abortion? Good luck on getting that in countries like the US, where many consider it murder, or plain against the law. Not to mention that it can be psychologically scarring to have to give up a baby.
Sure, it's not directly hurting anyone... except maybe our standards.
I have to seriously ask where you draw the line. Would you think that it's okay for a son to have sex with his mother? Or a daughter with her father? If no, why is it okay for siblings to have sex? They're also of the same blood, and from the same parents.
I think it's stupid that a friend would smack me upside the head (only half-playfully) for suggesting that I consider a certain distant cousin of mine whom I hadn't seen in nearly two decades easy on eyes.
I agree it's stupid. ZOMG thought crime!