A teenage boy underwent "unimaginable physical torture" before being drowned by his sister and her partner because they believed he was a sorcerer who was practising witchcraft, a court heard on Thursday.

Eric Bikubi and Magalie Bamu, both 28, killed 15-year-old Kristy Bamu in their east London flat after violently abusing him for several days, and repeatedly attacked the victim's two sisters, whom they accused of sorcery, the Old Bailey heard on the opening day of the trial.

Over four days Kristy, who was visiting his sister from France, was tortured with metal bars, wooden sticks, a hammer and a pair of pliers in a "prolonged attack of unspeakable savagery and brutality", the court was told.

After being denied sleep and food and having being repeatedly attacked, Kristy admitted to being a sorcerer in the hope that the violence would stop. He finally begged to be allowed to die.

On Christmas Day 2010 the defendants, who both deny murder, allegedly forced the boy and his siblings into a bath, submerging him in water. Pathologist reports revealed he suffered 101 injuries, and died as a result of drowning and the injuries.

When Kristy was found by paramedics in the eighth-floor flat in Forest Gate, his head, face, back and arms were covered in deep cuts and bruises, and several of his teeth were missing.

Kristy and four siblings were sent by their parents, from their home in Paris, to see their sister and his partner on 16 December. A few days passed "pleasantly enough", then Bikubi said some of the children were possessed by spirits and were having an evil influence on a younger child in the house, it is claimed.

"Despite her own siblings' denials that they were involved in witchcraft, Magalie Bamu joined her boyfriend in repeating these fantastic claims and participating in the assaults," said Brian Altman QC, prosecuting.

The court was told that over a period of days the pair, originally from the Democratic Republic of Congo, attempted to exorcise evil spirits they believed were in three of the children – Kristy, his sister Kelly, 20, and their 11-year-old sister, who cannot be named. Bikubi refused "to let them eat, drink or sleep for days, while the punishments became increasingly violent, with [the attackers] using the many implements found in the flat as weapons of torture", Altman said.

During their ordeal the siblings were forced to pray and chant throughout several nights and, in a "staggering act of depravity and cruelty", the defendants recruited sibling against sibling as "vehicles for their violence", said Altman.

Kristy became the focus of Bikubi's attention, the court heard. He allegedly struck the boy with a hammer in the face, knocking out his teeth; on another occasion he shoved a metal bar into the teenager's mouth, the court heard.

When Kelly attempted to hit her brother using something light, she was ordered to use a heavier implement. In a desperate attempt to prevent any further suffering, Kristy and his two sisters eventually admitted to being sorcerers, said Altman. "As Kristy's injuries became ever more severe he even pleaded to be allowed to die," he added.

When paramedics got to the flat just after midday on Christmas Day 2010, there was blood across walls, furniture, ceiling and floor.

Police found Kristy's brothers, Yves, 22, and a 13-year-old, and Kelly and her 11-year-old sister in the living room. "All were standing in the living room, hysterical, terrified and soaking wet. None spoke any English. both girls appeared to be injured."

A French-speaking paramedic asked Kelly to explain what had happened. "What she told him was a tale of horror," he said. She indicated an "armoury of weapons", saying Bikubi had hit her over the head with a metal bar and a piece of wood and pointed to a hammer she said he used to hit Kristy in the mouth. At one stage Bikubi smashed ceramic floor tiles over Kristy's head and jabbed him with a knife, she told the paramedic.

The jury heard an account of the last moments of Kristy's life. "Eventually, Bikubi took him into the bathroom, put him in the bath and started to run the water. Kristy was just too badly injured and exhausted to resist or to keep his head above the water," said Altman. "While Kristy succumbed to death, Bikubi put Yves, Kelly and the two other children into the bath with their brother and hosed them all down with cold water using a shower head."

Bamu called 999 saying her brother had drowned himself in the bath. Paramedics found Kristy's body on the bathroom floor, where Bikubi had tried to revive him.

The court heard that a series of calls were made to the children's parents in Paris during their four-day ordeal. One call on Christmas Eve lasted two and a half hours and between midnight and 7am on Christmas Day, 45 calls were made to Kristy's parents.

The court heard that in the early morning Bikubi told Kristy's father, Pierre Bamu, that he would kill his son if he was not collected. Told to speak to his father, Kristy reiterated the plea, saying: "Uncle Eric will kill me."

Pierre dismissed fears about the children because he could not imagine Bikunbi doing any harm to them, the court heard. But after the threat the couple tried, and failed, to get to London on Christmas Day. At about 8pm Pierre had a call from Kelly. "She told him, 'Dad, we are at the hospital, Kristy is dead', " said Altman.

The court heard that following neighbours' complaints of flooding, the council sent a plumber, who entered the flat 20 minutes before the emergency services were called. He saw "nothing untoward", thought nothing of splashing from the bathroom and left shortly after, the court heard.

The jury was told the belief in kindoki – the Lingala word for sorcery – was pervasive in the Democratic Republic of Congo. Expert witnesses in the trial would make clear there was "no provision in Congolese life and culture for the maltreatment and abuse of children", Altman said. "However, if the practice of kindoki is dislocated from the control of churches in the Congo, and the supervisory influence of religious leaders, then it may take on a feral and, indeed, evil character, as we suggest it did here."

Bikubi denies murder, but admitted manslaughter on grounds of diminished responsibility, and pleaded guilty to two counts of actual bodily harm. Magalie Bamu denies murder and two counts of actual bodily harm. The trial continues.