Dork Diaries: Sister Showdown (Dork Diaries #16) - Rachel Renée Russel

It’s the end of summer and Nikki has only 72 hours to completely reinvent herself before starting at Westchester Prep . . . But when disaster strikes and her bratty little sister Brianna gets hold of Nikki’s diary, her world is turned upside down. Will an unexpected emergency ruin Nikki’s fabulous start to the new school year or, worse, ruin her relationship with wacky BFFs, Chloe and Zoey, and cute crush Brandon?