(a) A person commits the crime of incest if, being 18 years of age or older, that person engages in sexual penetration with another who is related, either legitimately or illegitimately, as (1) an ancestor or descendant of the whole or half blood;
(2) a brother or sister of the whole or half blood; or
(3) an uncle, aunt, nephew, or niece by blood.
(b) Incest is a class C felony.