Incest is an "add-on" crime. Hardly anyone gets charged with just incest - usually it's rape or "something-or-other with a minor" with incest added to justify a harsher sentence. Its primary purpose is to punish child molesters.
Problem is, non-molesting incest happens a lot more than people want to admit. It's not too unusual for a couple of teenage siblings to experiment with each other. Usually it doesn't go as far as penetration, but even that happens a lot more than people think. And then they grow out of it and go on to live normal lives.
Incest laws turn these kids into criminals. It's not a deterrent - kids aren't going to let "it's illegal" stop them from doing secret things in private. And if the law gets involved - usually because a hyperconservative adult found out and reported it - it ruins lives.
The "must be 12 years old" part is for those kids, not for adults wanting to screw teenagers. It also allows the law to remove older kids messing around with elementary-school kids.
