The Steps — Rachel Cohn

Twelve-year-old Annabel thought Christmas break was going to be amazing. She'd planned to stay home in New York City with her best friend and do traditional things like go ice-skating in Rockefeller Center, hit the after-Christmas sale at Bloomingdale's, and scream with the TRL crowd at MTV in Times Square. But when her best friend bails, Annabel's mom decides it's high time Annabel visit her father and his new family in Australia.
Annabel is not pleased about traveling around the world to meet "the steps" -- twelve-year-old fashion-disaster stepsister, five-year-old stepbrother, and baby half sister -- but she's not going to waste this chance to steal her father back.