I have had similar experience as u r having while i was in 11th.
the only difference is the student was a boy in class 7th, and was son of my dad''d super boss.
try to think on these lines.
minimize your labor, give her a lot of homework and if she doesn't solve it, doesn't move up.
Don't use your brain much in explaining her theories, just use ur brain to check if any of her answers are correct.
Give her time once in a week.If possible, go and teach her in her house( you will be safer) and use sentences like, i am wasting my time with u. u need a full time tution. If her parents listen to it, its good. Make them feel like u r being harassed.
After a few classes, you can directly talk to their parents.
Beside, the best option is talk to her parents, tell them
"i am capable of teaching her, but she requires 5 times the time a normal student uses. Hence i am sorry but i can't give her that much time. i will surely help her after exams in holidays, till then please get her a real tutor."
Remember, her dad id ur dad's boss, so you need to be diplomatic to not to create problems on the dad level.
Lastly, i am going to share one important point i have learnt from my experiences.
"if its not business, its totally safe to abuse your boss with a smile."
Do tell me the results if you plan to implement it.
best of luck.