I've had success with a brightly colored chart I created and taped right up on the piano in front of us. Did she sit down and check her position? One sticker goes up. Is she listening to me? Another sticker. The sheet goes home so mom can see our progress. She improved dramatically.
... but will drop the activity immediately if I notice her attention straying...
...I'd advise against getting another authority figure for control. YOU must disciplin, otherwise, she'll never respect you, and she'll never do what you ask....