HaroldRowsdower said: Notice like that, though? that's a critical Westermarck Effect failure right there.
Westermarck Effect: is a hypothetical psychological effect through which people who live in close domestic proximity during the first few years of their lives become desensitized to later sexual attraction.
This means that people living together such as sibling become disinterested in sex with each other. The Westermarck Effect is theorized to take place from age 1-6. Again though it is a very loose theory, similar to the Oedipus complex.