Devalon Armstrong, 13, is accused of killing his 5-year-old sister Viloude Louis (pictured) with professional wrestling moves that he watched on TV.
The incident happened in the family's home in Terrytown, La., on Sunday afternoon.
Armstrong reportedly body-slammed his sister on the bed, hit her in the stomach and bashed his elbow into her several times.
Louis suffered broken ribs, internal bleeding and a lacerated liver, Jefferson Parish Sheriff's spokesperson Col. John Fortunato told The Times-Picayune.
After a 911 call from the home, paramedics found Louis unresponsive and she was declared dead soon after.
The girl's mother had apparently gone to the store while Armstrong was babysitting.
On Monday, Armstrong told police that he had been practicing World Wrestling Entertainment moves on the girl.
According to police, Armstrong said even when Louis told him that she was in pain, he continued the wrestling moves.
Police claim that Armstrong appeared to enjoy talking about the assault.
After a coroner investigation determined that she died of multiple injuries, Armstrong was arrested on Tuesday on suspicion of second-degree murder and is being held at a juvenile detention center.
However, the question of Armstrong being competent enough to stand trial has been raised and he will undergo an evaluation.
Source: The Times-Picayune