14 year old disrespectful and doesnt listen?

My niece is 14. She is basicly a good kid but has a mouth on her that just needs to be slapped shut. My sister doesnt know what to do with her. She talks back, talks rudely. She has been grounded punished, ect...But still doesnt make a difference - IMO my sister doesnt stick with the groundation hard enough cuz I know she is tired of listening to her whine and complain and just lets in to her. Well yesterday she severely pissed me off - I was giving her a ride and she whined and cried for a milkshake (shes severely over weight also so shes on a diet that she can have it) I didnt have any money and told her no that Im broke. She got all pissy and was whiiny then got an attitude and was like you have money the state pays for everything you have...It took alot for me not to b*tch slap her. I have 2 little kids to take care of and ran into some problems and moved here to be closer to family and I live in low income houseing, and I do work.