Detectives spoke with the girl's mother, Adlourdes Desvallons, 39, and determined the teen was babysitting the girl, Viloude Louis, at the time of the incident Sunday.
Devalon allegedly injured the girl while performing wrestling moves at their Terrytown apartment, The (New Orleans) Times-Picayune said.
He told investigators his sister told him he was hurting her, but he continued to slam, punch and elbow her.
Devalon said his sister had complained about a stomachache after he performed wrestling moves on her.
He called 911 a short time later after his sister stopped breathing, The Times-Picayune said.
She was transported to a hospital where she was pronounced dead.
Although she had no outward signs of injury, an autopsy revealed the girl died from severe blunt force trauma, The Times-Picayune reported.
Officials obtained a warrant and took Devalon into custody.
He was charged with second-degree murder and is being held at a juvenile detention facility.