Nice post. It's somehow concerning that this comes from a mormon though.
I am sneaky, a philosophical ninja.
Actually it's been claimed that there's both natural instincts and psychological reasons for incest taboo (brother-sister relationship).
I guess some studies showed that regardless of cultural views of incest, most people would be less attracted to other individuals based on having witnessed their mother showing care to the other person. Hence one may be adopted to a full sibling separated at birth, but not to a stepsibling.