In Utah, a person is guilty of third degree felony incest when, under circumstances not amounting to rape, rape of a child or aggravated sexual assault, he has sexual intercourse with a person whom he knows to be an ancestor, descendant, brother, sister, uncle, aunt, nephew, niece, or first cousin. The relationships referred to herein include blood relationships of he whole or half blood without regard to legitimacy, relationship of parent and child by adoption, and relationship of stepparent and stepchild while the marriage creating the relationship of stepparent and stepchild exists.