Three days before his jury trial was set to begin, David Brown changed his plea to guilty.
Brown is charged with attempted first-degree intentional homicide. The 15-year-old was to be tried as an adult, and had originally pleaded not guilty.
Brown shot his sister in the face last February and then beat her with a broomstick. She had reportedly been teasing him for missing the school bus.
According to Judge Robert Kinney, Brown is not changing his plea in exchange for a lighter sentence.
Brown's attorney says David changed his plea so his sister would not have to testify against him at his trial.
Brown will be sentenced Oct. 25.