Well, about the different rules, I undestand what you mean, you can check the my post when I checked the age of consent in USA.
In short, man, you have a mess, and if you know two young persons who want to have any kind of sexual activity, I would stronly advise they check with a specialised lawyer ! :-D
You even states that prohibit anything below 18 even if consensual and both below 18; states that have an age of consent, but then allow someone below the age of consent to consent if the partner is less than two or less than three years older, to the point that would accept consent from 13 year old if the partner is 14 or (not yet, if I recall well) 15; one in which one can have sex with someone below age of consent, if they are legally married (and how do you marry in a legally valid way if they are below the age to consent to marriage ?)...
But still, I doubt they will use so freely the "incest" label, the same way in which in USA they do not want to use the "statutory rape" label, although they apply widely in practice (and with the mess both you and I already referred to

).