How to deal with bratty niece?

I have to babysit her sometimes. she whines and throws a tantrum if she doesn't get her way. she thinks i'm "stupid" and "ugly" and doesn't listen to a word i say. (it's always "you're not the boss of me! you're just a kid!" (she's 4, i'm 14) when my brother put me and my dad both in charge to watch her. so she's especially bad when no adults are around, only me. she's already mean too. like a couple days ago i was babysitting her, and i had to help her with her bath. afterwards she started talking about her private area, then punched ME in the private area. and she flashed me twice. and she always stomps on the furniture. she doesn't listen to me when i tell her to stop, she either continues or misbehaves another way. so how do i deal with her when i babysit her again? i cant just stop babysitting her