A child watched a 15-year-old drown in a bath after being tortured over claims he was a witch, the Old Bailey has heard.
Kristy Bamu's sibling, who cannot be named for legal reasons, described how the teenager was attacked with weapons including a hammer before he died.
Kristy, from Paris, was found dead in a bath in a flat in Newham, east London, on Christmas Day 2010.
Eric Bikubi and Magalie Bamu, Kristy's sister, both 28, both deny murder.
Mr Bikubi has admitted manslaughter on the grounds of diminished responsibility, as well as two charges of causing actual bodily harm to Kristy's siblings.
Magalie Bamu denies murder and causing actual bodily harm to her siblings.
Part of a video of the account that the child witness gave to police was played in court.
Kristy and four of his siblings had gone to stay with their sister at her flat in December 2010.
While they were there, she and her partner Mr Bikubi accused them of being witches and trying to control another child, the jury heard.
Mr Bikubi found a pair or urine-soaked underwear in the flat's kitchen, and Kristy admitted that he had wet himself.
This triggered an outpouring of violence by Mr Bikubi, who believed the incident was due to witchcraft, the jury heard.
Mr Bikubi insisted that the siblings would have little food or sleep, the court was told.
'Mission to kill'
"He said we are going to do this for three days," the witness told the court.
Mr Bikubi later attacked Kristy with a dumbbell bar and a stick, threatening to kill him, the Old Bailey heard.
He forced the siblings to hold Kristy as he smashed four floor tiles over his head and later hit him with a hammer, the court was told.
Through fear Kristy admitted to being a sorcerer, and to having a mission to kill Mr Bikubi, Magalie Bamu, and another child, the jury was told.
The court heard Mr Bikubi forced Kristy to get into the bath and hosed him down with cold water but the teenager was too badly injured to get up.
Speaking through a French interpreter, the child witness said: "My brother was still not moving and Eric noticed that he wasn't moving.
"He took hold of him, putting him on the ground. He wanted to give him heart massage but it didn't work."
The case continues.