I think that the only reason, they made up Vincent in part three (as he actually doesn't even exsist in the book) is because FFC, wanted his daughter to have a love interest, and it would have been frowned upon if it had been any of the full blooded cousins.

Maybe FFC, having his daughter in the film, ruined the movie as he had to write the script around her stupid sub plot.
FFC's bias got in the way of what could of/should of been a great movie.