Brigette (Emily Perkins) cuts her hand and mixes her blood with Ginger's (Katherine Isabelle), to show her that they will be together forever, no matter what happens. The vial of wolfsbane (a cure for lycanthropy) Brigette and Sam (Kris Lemche) were going to use to cure Ginger is destroyed before they have a chance to use it. They don't make another before Ginger fully transforms and kills Sam. Trapped alone in their house with her now fully-transformed sister, Brigette gets Ginger to attack her, waiting for the moment to stab the same knife as before into the werewolf's heart, killing her.
Thanks, Rob T!
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