I'm not saying it was a right decision to cast his own daughter for leading role as Mary Corleone, but I'll understand it as he was experincing problems and knew it would cost alot more money to recast for that role. For an 18 year old she did her job as well as it can be expected from an 18. There were some scenes I liked and some I didn't. Also Francis likes to involve all family into the production of Godfather films and after losing his son having his own daughter be shot maybe he thought he could convey more drama to that moment.