What? Are you from Kentucky?
IC 35-46-1-3
Incest
Sec. 3. (a) A person eighteen (18) years of age or older who engages in sexual intercourse or deviate sexual conduct with another person, when the person knows that the other person is related to the person biologically as a parent, child, grandparent, grandchild, sibling, aunt, uncle, niece, or nephew, commits incest, a Class C felony. However, the offense is a Class B felony if the other person is less than sixteen (16) years of age.
(b) It is a defense that the accused person's otherwise incestuous relation with the other person was based on their marriage, if it was valid where entered into.
As added by Acts 1976, P.L.148, SEC.6. Amended by Acts 1977, P.L.340, SEC.86; P.L.158-1987, SEC.5; P.L.79-1994, SEC.16.
Looks to me like it is illegal no matter how old you and your relative are.However, once you are 18, and all parties are willing, the law (generally) has no limits.