Listen, Word to the Wise.
Unless she asked for it, knows about training, care, and the breed, do NOT get her a Jack Russel. I have one. They're VERY hyper dogs. Don't get one ESPECIALLY since she's 12! Kids don't do well with these HYPER dogs, also since they have school to tend to.
Jack Russels can be extremely destructive. They need CONSTANT attention as puppies, and need A LOT of training, LOTS of walks, and plenty of socialation and training.
Please reconsider unless the ENTIRE family is willing to pitch in and care for a dog. Jack Russels live from 8-15 years, can you handle one for that long?
Anyway, GOOD breeders will sell quality dogs from $400-600.
Please do research, and reconsider. Try asking her if she'd like a more quiet, less maintanence dog like a Clumber Spaniel. Children generally don't do well with caring for animals, unless it's something simple like a rabbit.
http://dogbreedinfo.com/ready.htm
http://akc.org
http://petfinder.com
Vet bills can go from $50-1000+. Food, toys, and other supplies can go to $100 also. Please, PLEASE get EVERYONE prepared. And make sure that your little sister CAN and WILL care for the dog. Words aren't as strong as actions.