(a) A person commits the offense of incest when he engages in sexual intercourse with a person to whom he knows he is related either by blood or by marriage as follows:

(1) Father and daughter or stepdaughter;

(2) Mother and son or stepson;

(3) Brother and sister of the whole blood or of the half blood;

(4) Grandparent and grandchild;

(5) Aunt and nephew; or

(6) Uncle and niece.

(b) A person convicted of the offense of incest shall be punished by imprisonment for not less than one nor more than 20 years.