Ashton Victoria. Confessions of a Teen Nanny

HarperCollin Young Adult, chick lit.
Sixteen-year-old Adrienne Lewis is in charge of eight-year-old Emma Warner, the youngest member of the snooty Warner family. Emma is an evil genius who has gotten all five previous nannies fired, and she's the good news. Because then there's Emma's half brother, Graydon, who goes to college, yet always seems to be lurking around waiting to hit on Adrienne. But worst of all is Emma's beautiful seventeen-year-old half sister, Cameron, whose reputation as a wild girl, a liar, and a user is known to everyone...everyone, that is, except Adrienne.