TYLER (February 2, 2012)—State District Judge Jack Skeen, Jr. sentenced a 16-year-old East Texas boy to 20 years in prison Thursday for the September 2010 shooting death of his older sister.

Mark Nathaniel "Nate" Anderson of New Chapel Hill pleaded guilty to murder Thursday in the slaying of his sister Amanda Anderson, 19, whose body was found at the family home near Tyler with four gunshot wounds to the head.

Skeen sentenced the boy under terms of a plea agreement with prosecutors.

Anderson was 15 at the time of his sister's death, but last June a judge ordered him tried as an adult.

Smith County District Attorney Matt Bingham said Anderson must serve 10 years in prison before becoming eligible for parole.

The motive for the shooting hasn’t been revealed.