Melissa says the Devil cheated her, but technically, she's the one who broke the contract.
Also, it's assumed at the beginning that Melissa is the daughter of the middle-aged farm couple, Luther and Molly. However, once it's revealed that Melissa is, in fact, the 127-year-old sister of the elderly woman, it begs the question, who the hell are Luther and Molly?
Quoth Paul Chaplin: "The people who are apparently her parents? It's unclear who they are. It's never really explained. They may just be walnut farmers who happen to own a witch."
Could be that they are her children or grandchildren... which only raises another question: If Melissa already has children, why does having sex with Jodie oldify her?
Love Conquers All? Melissa did say that Jodie could save her if he loved her, and break Satan's contract, and her previous relationships didn't involve true love.
I thought maybe they were Lucinda's children or grand children from a relationship with she had with someone in her 127 years, and she only went Axe Crazy when she got old. It would explain why they share in the responsibility of restraining and caring for Lucinda.
Although it does not, on the other hand, explain why only Melissa can calm her down, while Luther and Molly can only hold her back with brute force.