Unknown to the rest of their family, Tiffany and Matt were engaged in an incestuous sexual affair. Tiffany had wanted to kill her husband, so she took Matt's virginity and told him that they could continue their affair if he got rid of his father.
Due to early warning signs of being a psychopath, Amanda's concerned principal suggested that Tiffany bring Amanda to see Dr Vance.
One night, after everyone else gone to bed, Matt came into his mother's bedroom, where they had energetic sex. Amanda, having been unable to sleep, was on her way back from the bathroom when she heard Tiffany moaning. She crept up to the open door and witnessed Matt and Tiffany making love. This finally drove Amanda over the edge and she set the house on fire, killing Tiffany as well as Matt and Tamara.