It's one of those books that are in that sort of pre-YA category - the narrator is a 10 year old girl, and the book is mainly about her older sister, who is 13, pretty sure she's called Amanda, and who is obsessed with some popstar named Corey and the story basically centers around her wanting to go to see Corey in concert, her parents not allowing it, and her younger sister helps cover for her as she sneaks out to the concert.
I don't really remember many more details, apart from one scene where the younger sister blacks out Corey's teeth on a magazine cover... I think the book may have been part of a series, but can't be sure as I only read this one. It was probably written in the early-to-mid 90's. The front cover showed the sister sneaking out the front door and making a 'shh' gesture to the 10 year old, although I had the British version so I don't know if it's the same on other editions.