Elena Rendell, 17, is alleged to have shot her younger sister Christina in the neck as they fought in their Florida home.
Cops say that she grabbed a 9mm pistol from a shelf, pointed it at her sister and fired a single, fatal shot.
Sheriff’s office spokesman Deputy Chris Welborn said: "It appears that the two juveniles were at home and involved in some sort of argument."
Neighbors told The Pensacola Journal that Rendell then ran from the house, shouting for help, screaming that her sister, thought to be a foster sister had been shot.
Paramedics then rushed the girl to hospital, but they were unable to save her and cops picked up the youngster without incident.
Astonishingly, she only found out the news that her sister had died, during a court hearing for the shooting.
But she seemed unmoved, staring straight ahead as she was charged as an adult with manslaughter with a firearm, a second-degree felony.
The maximum sentence is life imprisonment.
Judge Ross Goodman rejected a plea from her brother to release Elena into his care and instead she was held on $300,000 bond.