Jessica is the perfect "girly girl with a mean streak" name to me so that's a good pick. All the Jessicas I've known have been brutal.
If you wanted to keep a tie to the past, I'd maybe use something like
Bella, or
Isabelle, that points at the femininity and Disney stuff, but isn't too obvious.
You could also use the name of other people or characters that share similar traits with your sister's character, like
Annie for
Annie Wilkes (also that ties into her real name as Misery was written by
Stephen King who also wrote
Christine), or
Millie or
Louise (referencing nurse
Mildred Ratched from One Flew Over the Cuckoo's
Nest, a famous character that symbolizes how people use the system to gain power and abuse others; she was played by actress
Louise Fletcher).