Ok, i need help with this babysitting problem?

Ok, so I babysit these two kids and one always gives me problems. I started out playing with them while my sister would babysit and now i babysot them now that im older. I am 14 cand the pain in the kid is twelve. She always undermines my authority, and is a real *****. and the mom thinks its fine. Like, th other day i babsat them and she was dribbling a basketball in the street and I asked her to stop and dribble on the sidewalk and becausse I dont feel comfortable with her in th street. please help.