Eric Bikubi and Magalie Bamu killed Magalie’s brother Kristy at their home in East London after torturing him for several days because they believed he was possessed.
Kristy drowned in a bath on Christmas Fay in 2010 during part of, what it was claimed, was an exorcism.
Bikubi had admitted to manslaughter on the grounds of diminished responsibility but the prosecution pushed for a murder conviction.
The pair will be remanded in custody for sentencing on Monday.
A statement from the family said: “”We will never forget, but to put our lives back into sync we must forgive.
“We take no comfort in the verdicts – we have been robbed of a beloved son, a daughter, a son-in-law.
“Kristy died in unimaginable circumstances at the hands of people who he loved and trusted. People who we all loved and trusted.”
Judge David Paget exempted the jury from having to pass judgement on any more cases for the rest of their lives such was the harrowing nature of the case.
“It is a case we will all remember,” said Paget.
“Court staff will speak to you and offer help to you.”
The jury had heard during the case that Kristy had begged to die after being attacked with knives, sticks, metal bars, a chisel and a hammer.
Kristy was murdered while he was visiting Bamu and Bikubi for Christmas. Bikubi accused Kristy of casting spells on his siblings, and forced the other children to join in with the attacks.
The prosecution argued that Bikubi had been mentally ill when he carried out the attacks, after a scan showed lesions on his brain.
Following the verdict, chief prosecutor Jenny Hopkins said: “His actions were nothing short of torture and he inflicted on the victims violence on an unimaginable scale,” she said.
“It has also been proven that his accomplice – Magalie Bamu – acted of her own accord.
“She willingly subjected her 15-year-old brother to extreme violence.”
Kristy’s family say they will take comfort from the case exposing the plight of children wrongly suspected of witchcraft or possession.
Image via Met Police