Kristy Bamu suffered 101 different injuries from being attacked with a chisel, a hammer and a metal bar, and was in such pain that he begged to die, prosecutors told the Old Bailey in London.
The victim had travelled with his two brothers and two sisters from Paris, where they lived with their parents, to stay with their older sister Magalie Bamu at her flat in Newham, east London.
Prosecutors said Magalie Bamu's partner, Eric Bikubi, had started the horror by accusing Kristy and his two sisters of witchcraft and Bikubi then launched a campaign of torture.
Kristy's siblings were forced to join in the attacks on him during their four-day ordeal before they were all placed in a bath and hosed down with cold water on December 25th, 2010, the court heard.
Paramedics were called when Kristy fell unconscious but he was pronounced dead and an autopsy found he died from a combination of being beaten and drowning.
"Kristy had been the victim of a prolonged attack of unspeakable savagery and brutality," prosecutor Brian Altman said.
"Wickedly, the defendants also recruited sibling against sibling as vehicles for their violence. In a staggering act of depravity and cruelty, they both forced the others to take part in the assaults upon Kristy."
Several calls were made to the children's parents in the French capital but their father did not realise what was going on, Altman said, adding: "He had sent his children on holiday, not to a torture chamber."
The defendants were both originally from the Democratic Republic of Congo.