I don't that incest, though disgusting, should come with a sex offender label that is going to follow you for life, especially between minors. Kids do weird poo sometimes if they get caught up in a moment.
If anything the definition of sex offender needs to be less broad. The majority of people, when they hear registered sex offender, think child rapist, or regular rapist, or some kind of dangerous person. Incest isn't innocent, but it's not dangerous unless your family is purposefully interbreeding. Even then it's only dangerous to your family. That should be illegal.
Now wherever these two move, they will have to put signs up in their yards and answer "I had sex with my sibling with I was a teen" whenever a neighbor asks them what they did. That's just embaressing.