I still say she needs a LOT of good exercise every day and training. You have not taken her to Obedience classes, have you? You and your family seem like you don't know how to begin to train Sassy... I'm sorry if that sounds harsh, I've tried rewording it but that's how I feel. A lot of people have this problem, thats why Obedience works! Obedience classes does not just train your dog, it trains YOU to train your dog. You can only get so much verbal advice on these boards, you also need to get physical and demonstrational advice from a lincensed dog trainer. It seems like there are soooo many issues here, and your parents do not want to help at all.

On top of signing up for Obedience classes, you should keep her on a long-line in the backyard if she refuses to listen. When she starts to pull towards the fence, tell her "Quiet" or "Leave It" and pull the lead towards you. Reward her when she starts to leave the fence with only a verbal command. I honestly don't think its fair for your parents to put her full training and behavior on a 12 year-olds shoulders. THEY NEED to pay for Obedience classes... if they're willing to pay hundreds for the actual dog, they *should* have no problem paying $100 for classes that will help everyone.