You know... I can't come up with a good argument against that...
Except for birth defects when siblings breed for generations. But it has to be for generations, and it only happens in parent-child and sibling relationships. It's been proven that cousins, grandparents, etc. won't breed defective children. There have been many societies where cousins have been marrying for centuries.
Then again, there's always Joffrey's psychopathic tendencies to consider. O.O