Stupidity: When Cathy, Chris and Carrie go into their late grandfather's room, they discover a copy of the grandfather's will that states that if it was ever discovered that Corrine had children, she would lose everything. Had they taken the will to their mom's wedding and showed it to her fiance, they would have been able to expose the scam much quicker. Instead, they take the dead mouse Fred with them and show it to everybody. What good would a dead mouse do when a will has more leverage?
Stupidity: When questioning the heavyset witness after his having seen Costner in the hallway, there is no mention of the most obvious identifiable feature...the pearly white Navy uniform he was wearing. Instead, Costner is described as being a common "everyman."
Suggested correction: We never see them put the will away. It's possible one of them had it in their pocket during the wedding scene, but they just never showed it because A. Showing the will would have given motive for their mother to want to kill them, but not have proved that these children were hers, and B. The dead mouse is a more visceral, hate-fueled move that matches Kristy's anger.