RCW 9A.64.020: Incest.

(1)(a) A person is guilty of incest in the first degree if he or she engages in sexual intercourse with a person whom he or she knows to be related to him or her, either legitimately or illegitimately, as an ancestor, descendant, brother, or sister of either the whole or the half blood.

(b) Incest in the first degree is a class B felony.

(2)(a) A person is guilty of incest in the second degree if he or she engages in sexual contact with a person whom he or she knows to be related to him or her, either legitimately or illegitimately, as an ancestor, descendant, brother, or sister of either the whole or the half blood.

(b) Incest in the second degree is a class C felony.

(3) As used in this section:

(a) "Descendant" includes stepchildren and adopted children under eighteen years of age;

(b) "Sexual contact" has the same meaning as in RCW 9A.44.010; and

(c) "Sexual intercourse" has the same meaning as in RCW 9A.44.010.