Also, let's not forget that under islamic law she was his niece, since he had formally pledged brotherhood with the girl's father before marrying her when she was only six. Boinking a nine year old may be acceptable under islam, but boinking your niece is not. One of numerous 'islamic laws' he repeatedly violated. Perhaps the film should be retitled: Muhammad! The Incestuous Pedophile!, coming to a theater near you!!! (two stumps up!)
According to the Shariah, if a girl is a minor (did not attain puberty), she may be given in marriage by her father. When she attains puberty, she has the right to maintain the marriage or discontinue the marriage. There is no age limit to be intimate with ones wife even if she is a minor.
Additionally, it is permisible (under Islam) to marry your first cousin:
1. You are allowed to marry your first cousin. There is absolutely nothing wrong in this.