(1) Lilly’s sister, Anne Kincaid.
(2) Lilly and Anne were playing a prank on the boys. Anne made them chase after an imaginary killer, then stabbed her sister for real.
(3) The fake knife had its handlegrip facing the feet, while the real knife was turned the other way.
The police noticed the discrepancy between Mark’s and Larry’s description of the knife’s position in Lilly’s chest and that shown in the crime scene photos. Someone had obviously removed the knife and reversed it. Or stuck in a new knife, one with a blade.
Anne was brought in for questioning. Eventually the 17-year-old confessed. For years, she had been jealous of her sister. Lilly got everything: praise and attention and, most galling of all, a first rate education that their parents had saved years for.
It was Lilly who had thought up the prank. They would lure their boyfriends into the old mansion and then scare them with a murder, just like in the movies. Anne instantly saw how this fake murder could be turned into a real one. Lilly and their parents would pay for their favoritism. And Anne would get to go to Princeton.
During their exploration, Lilly tiptoed away and set up the grisly scene, smearing herself with cat blood and sticking the broken-off knife into her blouse. Anne made sure the boys didn’t get too close to the “body”, at the same time making sure that she got a sufficient amount of blood on herself.
The only thing Anne needed now was a few moments alone with her sister. As soon as she’d tricked the boys into running off, Anne produced a real knife and stabbed Lilly to death. The second scream also came from Lilly, and this time she wasn’t acting. In the 30 seconds it took them to run back upstairs, Anne had wiped off the fake knife and thrown it out the window.