A sister of a 15-year-old boy allegedly tortured and drowned because his tormenters were "obsessed" with witchcraft has described the last moments of his life.

Kelly Bamu, 21, struggled to contain her emotions at the Old Bailey as she was shown a series of weapons that the court heard were used to abuse her, her brother and her sister because Eric Bikubi and Magalie Bamu, both 28, believed they were witches.

Kristy Bamu, 15, was found dead in a bath on Christmas Day 2010 in a blood-covered flat in Forest Gate, east London. He had 101 injuries and was covered in deep cuts and bruising. An "armoury" of weapons allegedly used to abuse him was found in the flat, the court was told.

He and four siblings were visiting their sister Magalie Bamu in London from Paris for the Christmas holidays when Bamu and her partner became convinced they were witches, the court heard.

Kelly described how on Christmas Day, after several days of abuse, Bikubi – who, like Bamu, was born in the Democratic Republic of the Congo – had dragged Kristy into the bathroom by his clothes, put him in the bath and filled it with "very cold" water.

"Eric asked Kristy if he wanted to die. Kristy said: 'Yes, I want to die.' That's the last thing I heard from my brother," she said.

Pointing at her sister in the dock she said: "And she did nothing, she didn't even call the emergency services. I was the one who said call the emergency services."

Bikubi forced Kelly, her brother Yves, a younger sister and another child of the family into the bath with Kristy, who she said was under the water.

He then doused them in cold water from the shower head and made them chant, she told the court.

"Kristy was no longer moving, he was not moving at all," she said.

Indicating Bikubi, she said: "He says that he was not in his right mind but he was perfectly aware of what he was doing."

Addressing both defendants she shouted: "You don't know what you've done, you two. Did you ever think of your parents, of Mum and Dad?"

On another occasion she shouted: "You're still breathing, you're still alive … Why have you taken a life?"

Earlier Kelly detailed the extent of the abuse her brother had suffered. Shown a metal bar, Kelly told the court that during several days of extreme violence Bikubi had used the bar to hit Kristy about his body and head, and had repeatedly shoved it into his mouth, causing a tooth to dislodge.

"Eric shoved it as hard as he could with such force that it broke his tooth," she said. "Eric said: 'Get on your knees, get on your knees.'"

Asked if Kristy had voluntarily opened his mouth, she said: "Kristy had opened his mouth, he said it hurt, but [Eric] didn't care. He did it a number of times. He's sick," she said.

Kelly said that during the violence – which was also directed at her and a sister but focused on Kristy – the siblings were recruited to harm the 15-year-old.

Kelly was told to get some pliers and twist Kristy's ear, she told the court. "As if that wasn't enough, all this blood all over the house," she said. But Kelly was reprimanded when she did not do enough to hurt her brother. "I couldn't bring myself to do it, but he said if I didn't do it 'I'm going to do the same thing to you'."

She described her sister Bamu taking over: "Then Magalie took the pliers and she twisted the ear as hard as she could." Turning to her sister in the dock, she said: "You're an absolute disgrace."

Kelly went on to describe how Bikubi had placed a light bulb in her mouth, held a kitchen knife to her chest and put his hand around her throat, before doing the same thing with Kristy and her sister, who cannot be named for legal reasons. "He was intending to kill us, all of us," she said.

She also described how bottles were smashed over her brother's head, and Eric headbutted and punched him. "Eric just went for Kristy," she said. "Kristy was a child, he was 15 years old. His face was completely disfigured. Kristy, he suffered. And Magalie did nothing [to stop it] so both of them are guilty."

She later added that in a moment during the abuse Kristy had been allowed to sleep but Bamu had entered his room and said he was still practising witchcraft. "So [Eric] got us into the sitting room and it all began again," she said.

She said her brother had been forced to drink urine and the siblings were told to clean up the blood in the flat before his death. "There was blood everywhere," she said.

As Kelly told the court that tiles were smashed repeatedly over her brother's head, Magalie Bamu began sobbing loudly in the dock. "Eric took the tiles and then he hit his head again and again. Kristy was already tired by this point and couldn't take any more blows. But that wasn't enough for Eric. He wanted him to die," she told the court.

The tiles had been smashed the day before Kristy was drowned in a bath, she said. She added that the pair had repeatedly phoned their parents in Paris, and when Kristy was close to death Bikubi had told their father, Pierre Bamu, that if the teenager was not collected from the house he would kill him.

"Kristy was already on the edge. I could see in his eyes that he was already leaving us. Eric had the cheek to call our father and said: 'If you don't come and pick up Kristy and the children I'm going to kill them,' and that's what he did."

During several calls to the siblings' parents in Paris, Magalie Bamu denied the abuse, said Kelly. "They told me to say to my mother that I confessed I was a witch. I made it all up and they believed it." She added: "My mother didn't even understand anything I was telling her."

During one conversation, Bikubi was sat next to her on the couch with his fists clenched, she said. Kristy also spoke to his mother but did not tell her about the abuse. "Those blows, he'd accepted them, and when he was one the phone he didn't even say: 'They have been hitting me, I've been receiving all these blows.'"

All three told their mother they had confessed to being witches, she said. "We all made up these stories because these two were obsessed with it," she told the court.

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