incest

Definition from Nolo’s Plain-English Law Dictionary

Sexual contact between close blood relatives, including brothers and sisters, parents and children, grandparents and grandchildren, or aunts or uncles with nephews or nieces; 18 states also include copulation or cohabitation between first cousins in the definition of incest. Incest is a crime in all states, even if consensual by both parties.

Definition provided by Nolo’s Plain-English Law Dictionary.