Reading "The World of Ice & Fire; the untold history of Westeros and the Game of Thrones", a pretty good case is made that
Cersei and Jaime may actually be the Mad King's bastards.
This would go a long way to explaining certain qualities that both Cersei and Jaime possess.
Cersei certainly seems to have the whole crazy hostile and insanity thing going down, while Jaime is the best swordsman of his age, much like many earlier Targaryen.
More importantly, this also seems to suggest that the whole deal with incest
might actually be something biologically linked to the Targaryen and not just something cultural. Worth noting is that in the history of the Targaryen, several of them end up falling in love with their siblings despite their parents attempting to prevent that from happening.
Joffrey certainly seems to have the insanity down, but with the earlier spoiler in mind, does Joffrey ever show any incestuous interest in Myrcella?