Eric Bikubi, 28, and Magalie Bamu, 29, denied killing Bamu's 15-year-old brother, Kristy, who drowned in a bathtub on Christmas Day 2010 during a purported exorcism, the BBC reported.
The couple, originally from the Democratic Republic of Congo, are scheduled to be sentenced Monday.
Bikubi attempted to plead guilty to manslaughter with diminished responsibility, but prosecutors rejected the motion.
"Kristy died in unimaginable circumstances at the hands of people who he loved and trusted," his family said in a statement. "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."
Bamu had testified she was "very Westernized" and blamed the obsession with witchcraft on Bikubi, The Guardian said.
"Eric sometimes went off. He would talk about witchcraft, talk, talk, talk, then stop," Bamu said.
"I would be listening because that's the best thing to do when he's going on about something. When he gets it out of his system, you carry on with life. I don't have any views because I don't believe in it. To me it does not exist."
Judge David Paget said the case was so "harrowing" that jurors would be exempted from further jury duty for the rest of their lives.