LANCASTER – A Lancaster County teen pleaded guilty, but mentally ill to a charge of third-degree murder in court regarding the stabbing of her sister on Feb. 22, 2021.16-year-old Claire Miller will serve 12 ½ to 40 years in prison after a hearing before Lancaster County Judge Jeffery Wright. The judge determined Miller to be mentally ill, but not mentally disabled, accepted the plea, and imposed sentence. Miller was 14 at the time she stabbed her 19-year-old sister, Helen. Miller called 911 and reported that she had killed her sister to police at her family’s home.