A man and woman who tortured and killed a 15-year-old boy because they believed he was a witch have been found guilty of murder.

Eric Bikubi and Magalie Bamu, both 28, killed Magalie's brother Kristy Bamu in their east London flat after violently abusing him for several days because they believed he was possessed by evil spirits.

In a statement read out at the Old Bailey, the murdered boy's family said they were able to forgive his killers, but not able to forget the "terrible" events. "Kristy died in unimaginable circumstances at the hands of people who he loved and trusted. People who we all loved and trusted," his father, Pierre, wrote on behalf of Kristy's relatives and friends, including his mother, Jacqueline.

"Christmas, a festival of joy and Jacqueline's birthday, will always be scarred by these terrible events."

Bikubi and Bamu also accused two of the teenager's sisters of witchcraft and were found guilty of causing actual bodily harm after abusing them for several days. Bikubi and Bamu were remanded in custody to be sentenced on Monday.

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

A few days after Kristy arrived in London on 16 December 2010, Bikubi, a man he thought of as his uncle, began to accuse him of witchcraft after he wet his pants.

After being denied sleep and food and having been attacked with a number of weapons, 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, when Kristy was weakened as a result of the beatings, Bikubi forced the boy and his siblings into a bath. He doused them with freezing water and, as the bath filled up, Kristy was submerged and drowned.

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. Pathologist reports revealed he suffered 101 injuries and died as a result of drowning and the injuries.

Bikubi, originally from the Democratic Republic of the Congo, carried out a series of increasingly violent attacks over a period of days against Kristy, whom he accused of having an evil influence over another child in the family.

He forced Kristy and two of his sisters who he also believed were witches – Kelly, 20, and their 11-year-old sister, who cannot be named – to pray, denying them food and sleep in an attempt to "remove the kindoki", the Lingala word for witchcraft.

The trial was told the attacks began with beatings and intensified until Bikubi – helped and encouraged by Bamu – was using an "armoury of weapons" found in the flat. He beat the child with a metal bar used for weights, shoved a metal bar into the teenager's mouth and struck Kristy with a hammer in the face, knocking out his teeth. Bamu also beat him, taking a pair of pliers and wrenching his ear.

Giving evidence through a French interpreter, the boy's older sister Kelly, now 21, said the pair were fixated on the idea that the three siblings were practising witchcraft. "It was as if they were obsessed by that and then it became absolutely unbearable," she told the court. "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."

In a "staggering act of depravity and cruelty", the defendants recruited sibling against sibling as "vehicles for their violence", with Kristy's brother forced to stand guard to make sure he did not escape. Just before he died, two of the younger children were made to clean up Kristy's blood, which covered the flat.

The court heard that Kristy's parents, who were set to join their children a few days after Christmas after remaining in Paris because of work commitments, spoke to Bikubi, Bamu and finally their son on the day he died. Bikubi told Pierre Bamu, Kristy's father, that his son was possessed by witchcraft and if he was not collected immediately he would kill Kristy.

Kristy also spoke to his father on the telephone and warned him that Bikubi would kill him.

Pierre had dismissed fears about the children because he could not imagine Bikubi doing any harm to them.

On Christmas Day, after these phone calls, his parents attempted to get a last-minute hire car but failed, and instead decided to arrive as planned two days later. A few hours later, they received a call from Kelly to tell them Kristy was dead.

In the statement to the court, Pierre Bamu wrote: "We were always fond of Eric and regarded him as a son. We were proud that he would call us Mum and Dad. As a family we planned our futures together and Eric and I were to open a restaurant in London together as a legacy for our family."

The statement continued: "The pain of Kristy's death is something which cannot be measured or calculated. Kristy was a fine young man, kind and considerate, much loved by his family and friends. We saw that he was becoming a man. We hoped that he would work with me in my carpentry business and one day take over. Kristy was also a role model to his siblings."

The court heard that 20 minutes before Kristy died, the council sent a plumber to the flat who heard splashing in the bathroom, but nothing else suspicious. Neighbours told the court they heard angry shouting and continuous music throughout the night before Kristy died.

Neighbour Umar Anzaar told police: "I was aware of a lot of noise coming from the flat – it sounded like a fish market. There were a lot of people shouting, and the voices sounded angry. The noise was so loud that I shut the window."

The court heard that Kristy was put in a bath and doused with cold water. As the bath filled up, his siblings were also forced into the tub and Kristy became submerged in the water. Kelly noticed he was not breathing and Bikubi attempted to resuscitate the teenager. Bamu called the emergency services, but the child was dead by the time paramedics arrived.