From a marriage perspective, incest is the marriage between close or legal relatives. Incest is not legal in the U.S.
In the United States, every state prohibits you from marrying any of your ancestors or descendants including your brother, your sister, your half-brother, your half-sister, your aunt, your uncle, your niece, your nephew, your mother, your father, your grandmother, your grandfather, your great-grandmother, your great-grandfather, your child, your grandchild, or your great-grandchild.
Some states have additional prohibitions concerning marrying your adoptive brother, your adoptive sister, your step-mother, your step-father, your former step-mother, your former step-father, your mother-in-law, your father-in-law, your former mother-in-law, your former father-in-law, your adoptive mother, your adoptive father, your former adoptive mother, your former adoptive father, your step-child, your former step-child, your adoptive child, your former adoptive child, your daughter-in-law, your son-in-law, your former daughter-in-law, your former son-in-law, or your cousins. More on cousin marriages
Also Known As: Prohibited marriage