He had refused to admit to sorcery and witchcraft and his punishments in a ''deliverance'' ceremony became more horrendous.
He had come to London with his two brothers and two sisters from their home in Paris to visit Magalie during the Christmas holiday in 2010.
But Bikubi turned on them, accusing them of bringing kindoki - or voodoo - into his home.
He forced them to pray for deliverance for three days and nights and deprived them of food and water.
The sisters, aged 20 and 11, were beaten along with Kristy, but escaped further attacks after ''confessing'' to being witches.
Kristy was singled out after wetting his pants. He was struck in the mouth with a heavy bar and hammer, knocking out his teeth.
Ceramic floor tiles and bottles were smashed on his head and and a pair of pliers used to twist his ear.
The terrified siblings, who also included a 13-year-old boy and an autistic brother aged 22, were made to join in the torture.
At one point, Bikubi told the youngsters to jump out of the window to see if they could fly, the court heard.
They looked to their older sister to save them, but instead Magalie encouraged Bikubi and beat Kristy until he also confessed to witchcraft.
Sister Kelly, now 21, broke down several times in court as she relived the terror.
She said: ''They started talking about kindoki, witchcraft and this and that.
''It was as if they were obsessed by witchcraft and then it became absolutely unbearable.
''They asked if we were witches. I repeated again and again and again that we were not witches.
''I did not know what was going on in their minds. They decided we had come there to kill them.''
Kelly added: ''Kristy asked for forgiveness. He asked again and again.
''Magalie did absolutely nothing. She didn't give a damn. She said we deserved it.''
Calling Magalie ''an idiot'', Kelly said: ''I am sure she still believes even to this day that we are witches.
''I have no pity for her. She had no pity for us.''
At the end of their ordeal, they were all placed in the bath to be hosed down in cold water with a shower head.
Brian Altman QC, prosecuting, said: ''It was only when he realised that Kristy was not moving that Eric Bikubi stopped what he was doing and pulled him from the water. By then it was too late.''
Kristy had 130 separate injuries and died from a combination of being beaten and drowning.
Paramedics called to the eighth-floor flat tried to save Kristy but he was already dead.
In the blood-spattered living room, police found Kristy's brothers and sisters.
''All were standing in the living room, hysterical, terrified and soaking wet,'' said Mr Altman.
Items found in the flat had been used as ''weapons of torture''.
He said: ''In a staggering act of depravity and cruelty, they both forced the others to take part in the assaults upon Kristy.
''The children had no option other than to do as they were told, or risk the same violence to themselves.
''As Kristy's injuries became ever more severe, he even pleaded to be allowed to die.
''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.
''Kristy had been the victim of a prolonged attack of unspeakable savagery and brutality.
''Kristy was killed in the name of witchcraft. It is hard to believe in this day and age anyone can believe someone was practising witchcraft.''
Several calls were made during their four-day ordeal to the children's parents in Paris, but they did not realise what was happening.
''Mr Bamu had sent his children on holiday, not to a torture chamber,'' added Mr Altman.
Two years before, family friend Naomi Ilonga, 19, was made to cut off her long hair by the couple to ''release witchcraft'' after being caught biting her nails.
The jury was told that in the Democratic Republic of the Congo, where the defendants were originally from, witchcraft was practised in Christian churches.
But out of the church's control ''it may take on a feral and indeed evil character, as we suggest it did here'', Mr Altman added.
Bikubi admitted manslaughter on the grounds of diminished responsibility caused by brain damage, but this was rejected.
He also pleaded guilty to two counts of causing actual bodily harm to the girls. Magalie denied the assaults but was found guilty.
Detective Inspector Paul Maddock said later: ''It's an almost unprecedented scenario where siblings are murdering another sibling.
''The family have been very positive, they've pulled together remarkably well.
''They were more than willing to give evidence and make sure the perpetrators were dealt with appropriately.''
Scotland Yard has investigated 83 cases involving abuse resulting from ritualistic or faith-based beliefs, and brought 17 prosecutions, over the last 10 years.