D.C. Code § 22-1901

If any person in the District related to another person within and not including the fourth degree of consanguinity, computed according to the rules of the Roman or civil law, shall marry or cohabit with or have sexual intercourse with such other so-related person, knowing him or her to be within said degree of relationship, the person so offending shall be deemed guilty of incest, and, on conviction thereof, shall be punished by imprisonment for not more than 12 years. In addition to any other penalty provided under this section, a person may be fined an amount not more than the amount set forth in § 22-3571.01.