(a) A person commits incest if he or she marries or engages in sexual intercourse with a person he or she knows to be, either legitimately or illegitimately, any of the following:
(1) His or her ancestor or descendant by blood or adoption.
(2) His or her brother or sister of the whole or half-blood or by adoption.
(3) His or her stepchild or stepparent, while the marriage creating the relationship exists.
(4) His or her aunt, uncle, nephew or niece of the whole or half-blood.
(b)(1) Incest is a Class C felony.
(2) Where the victim is under 17 years of age on the date of the offense, incest is a Class A felony.