This is Graceanne's Book — P.L. Whitney

The story is told by a nine-year old boy, Charlie, who observes with an encompassing awe a pivotal year in the life of his older sister Graceanne. She's loud, intellectual and a ruthless physical and psychological daredevil, a girl whose ferocious exploits are the stuff of local legend and the stuff of all that Charlie aspires to be. He narrates Graceanne's painful passage into teenage, a passage made tempestuous by their violent mother.