The biggest flaw in the Mary character was a combination of bad writing, nepotism and poor acting. The bottom line is the movie fails when Mary is killed because the viewers never felt all that much affection for her.

The character was a shallow, spoiled and temperamental daughter. We clearly see how much Michael loved her, but it is not possible to believe Vincent fell for her as hard as he did, and when she died we did not share the pain of her family and of Vincent.