As for Washington, I doubt the fact that it was his sister had anything to do with the sex offender part of the case. It was most likely due to his being 16 and her being 15, which is probably a minor under Louisiana law. So I would imagine under Louisiana law that Washington was considered an adult and his sister was considered a minor; and that is what caused him to become a registered sex offender. And any other 16 year old who sleeps with a 15 year old in Louisiana would also have to register as a sex offender if caught. As an aside, the fact that you would be labeled a sex offender for the rest of your life for that is also ludicrous in my opinion.
PROH SEXUAL CONDUCTthat's pretty general sounding, but I'd guess it's more involving your ya ya and sisterhood, than the age thing.
hang on....I forgot I'm on the internet....let me just stroll over to the library.........
25.02. PROHIBITED SEXUAL CONDUCT. (a) A person commits
an offense if the person engages in sexual intercourse or deviate
sexual intercourse with another person the actor knows to be,
without regard to legitimacy:
(1) the actor's ancestor or descendant by blood or
adoption;
(2) the actor's current or former stepchild or
stepparent;
(3) the actor's parent's brother or sister of the whole
or half blood;
(4) the actor's brother or sister of the whole or half
blood or by adoption;
(5) the children of the actor's brother or sister of
the whole or half blood or by adoption; or
(6) the son or daughter of the actor's aunt or uncle of
the whole or half blood or by adoption.
(b) For purposes of this section:
(1) "Deviate sexual intercourse" means any contact
between the genitals of one person and the mouth or ###### of another
person with intent to arouse or gratify the sexual desire of any
person.
(2) "Sexual intercourse" means any penetration of the
female sex organ by the male sex organ.
© An offense under this section is a felony of the third
degree, unless the offense is committed under Subsection (a)(6), in
which event the offense is a felony of the second degree.