I wonder why people always use Alabama for this joke when Alabama is one of the states where incest is actually ILLEGAL.
Maybe it's the result of people from states with legalized incest trying to deflect attention away from themselves.
Incestuous
marriage is illegal in all 50 states and incest itself in 47. (I've seen a couple of links that say incestuous marriage is allowed in Ohio as well as another that says incest is banned there, so do your own research.)
Wikipedia
Incest is legal in Ohio (excluding "parental figures"), New Jersey, and Rhode Island. Some states define the restriction so narrowly that that if it isn't penis in the vagina it doesn't count. For example, Florida outlaws: "Marriage, sexual intercourse (defined as the penetration of the female sex organ by the male sex organ, however slight; emission of semen is not required)." Maine is very similar. So apparently in Florida and Maine your sister can legally give you a BJ or take it anally and it isn't a crime.
On the other end, Massachusetts is very broad on what they outlaw: "Marriage, sexual intercourse, sexual activities (including but not limited to, oral or anal intercourse, fellatio, cunnilingus, or other penetration of a part of a person's body, or insertion of an object into the genital or anal opening of another person's body, or the manual manipulation of the genitalia of another person's body)"
In Alabama incest is a Class C felony and includes those related by adoption as well as blood.