The sister of a teenager allegedly tortured and killed because his tormenters believed he was practising witchcraft broke down in the witness box as she recalled the abuse.

In angry scenes at the Old Bailey Kelly Bamu, now 21, confronted her sister Magalie Bamu and Bamu's boyfriend Eric Bikubi, both 28, accusing them of killing her 15-year-old brother after torturing him for several days, and said they should undergo the same fate.

Kristy Bamu was found drowned in a bath in an eighth-floor flat in Forest Gate, east London, on Christmas Day 2010. He was covered in deep cuts and bruises and had several teeth missing.

The pair also accused Kelly and her younger sister, who cannot be named for legal reasons, of witchcraft, attacking them with metal bars and fists, the court heard.

The siblings had travelled to London on 16 December from Paris for the Christmas holidays, along with two other brothers, but after a few days Bikubi and Bamu began accusing Kelly, Kristy and their sister of being witches, the court heard.

Over four days they repeatedly attacked the siblings, focusing on Kristy in particular, beating him with an "armoury of weapons" including a hammer, chisel, metal bars and pliers in order to exorcise evil spirits. The court heard he was found with 101 injuries and had "begged to die" to avoid more beatings.

Giving testimony through a French interpreter Kelly said they pair had become fixated with the idea that the three siblings were practising witchcraft after Kristy wet his underpants.

"Magalie and Eric said that the pants that had been weed on were bewitched," she told the court.

The pair accused the siblings of trying to harm them and another child of the family in the house, she said. "It was as if they were obsessed by that and then it became absolutely unbearable," she said.

"I repeated again and again and again that we were not witches that we had come there to spend Christmas as a family together. But I don't know what was going on in their minds. They decided we were there to kill them."

The siblings were then allegedly forced to say prayers throughout the day and night in Lingala, the language of the Democratic Republic of the Congo where both defendants were born, to "remove the kindoki", the Lingali word for witchcraft. They were denied food and sleep, and were prevented from going to the toilet.

"At first we fasted, then we prayed. We also had a vigil," she said.

Asked what that vigil entailed, she said: "It means you don't sleep and you pray and pray and pray."

But the violence against the two sisters and their brother soon escalated. Kelly described how Kristy had begged to be allowed to go to the toilet during the prayers, and after he was forbidden from doing so wet his pants on the couple's new sofa.

Bikubi tried to force the three to admit to being witches, she told the court. "He said since you don't want to tell the truth I'm going to get the truth out of you with a stick."

In highly charged evidence during which she repeatedly shouted at her sister who kept her head bowed, Kelly said her sister punched her in the face and did nothing to stop the vicious beating of her brother.

"She doesn't realise what she did. Kristy asked for forgiveness, again and again and again. Magalie did nothing at all [to help], she didn't lift a finger."

She added: "I am sure she still believes, even to this day, that we are witches. I begged Magalie and I said: 'We haven't done anything and you're going to let your boyfriend hit us and he has no right to do so.' She didn't give a damn."

Breaking down in tears, and struggling to continue to give evidence, she said: "It is very traumatic to see these things. [Kristy's] face was completely disfigured. Eric, he just carried on and on and Magalie she saw it all."

Pointing at her sister in the dock, she shouted: "Kristy is dead. He's not here anymore. They should have to undergo the same thing as happened to Kristy. Magalie deserves to die for what they did to us. I have no pity for her. She had no pity for us."

During the abuse the three said they were witches in the hope that the beatings would stop, and at one point she and her sister even accused Kristy of witchcraft "because we wanted them to stop beating us". But the pair were unrelenting, she said.

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.