>>17 "Laws vary by state, but generally, a person commits incest if he marries or engages in sexual intercourse with a person he knows to be, either legitimately or illegitimately:
His ancestor or descendant by blood or adoption; or
His brother or sister of the whole or half-blood or by adoption; or
His stepchild or stepparent, while the marriage creating the relationship exists; or
His aunt, uncle, nephew or niece of the whole or half-blood.
In some states incest also includes copulation or cohabitation between first cousins, but the majority of jurisdictions permit marriage between such cousins."
http://definitions.uslegal.com/i/incest/