The boy had complained of stomach and chest pains Tuesday and died after he was taken to Mount Hagen hospital, Papua New Guinea's The National reported Thursday.
The family accused three women of using sorcery to kill the boy, and 20-year-old Kepari Leniata allegedly admitted she was responsible.
The relatives tortured the woman with a hot iron rod, stripped her naked, bound her hands and feet, and threw her onto a pile of burning tires in front of a crowd, the newspaper said.
Paramedics arrived on the scene, but were unable to save Leniata, who was the mother of an 8-month-old girl.
David Piso, head bishop of Gut Nius Lutheran Church, condemned the killing.
"Sorcery and sorcery-related killings are growing and the government needs to come up with a law to stop such practice," he said.
Police said they are treating the case as a murder and plan to charge those they know were responsible. The lives of the two other women accused of sorcery were spared.