Michael Gilbert, 26, was kept as a slave and tortured before being killed and decapitated then dumped in the Blue Lagoon in Arlesey, Bedfordshire, last year.
Today James Watt, 27, his girlfriend, Natasha Oldfield, and his brother's girlfriend, Nichola Roberts, were unanimously convicted of Mr Gilbert's murder by a jury at Luton Crown Court.
James's brother, Robert Watt, 20, and his mother, Jennifer Smith-Dennis, 58, were also found guilty of familial homicide.
A third brother, Richard Watt, 25, previously admitted familial homicide and perverting the course of justice.
Speaking after the verdicts today, Mr Gilbert's family said they were pleased with the result but nothing would bring Michael back.
During the trial, the court heard how "vulnerable" Mr Gilbert was treated as a slave by the family.
Prosecutor Stuart Trimmer QC said he was hit with bats, shot, and stabbed. Beatings were often filmed on mobile phones.
On one occasion he was made to goad a large pet lizard until it attacked him with its tail, and Oldfield's diary included plans for a game show where contestants would pay £5 to slap him and £25 to headbutt him, the court heard.
Mr Gilbert escaped several times but they would track him down and force him to come back.
Today, the jury unanimously convicted James Watt, from Chertsey Close, Luton, Bedfordshire, of murder. He had already admitted familial homicide and perverting the course of justice.
Robert Watt, 20, was convicted of familial homicide and had already pleaded guilty to perverting the course of justice. He was acquitted of murder earlier in the trial.
Richard Watt's girlfriend, Nichola Roberts, 22, and Natasha Oldfield, 29, were both found guilty of murder and perverting the course of justice. Oldfield was also convicted of assisting an offender.
The brothers' mother, Jennifer Smith-Dennis, 58, was found guilty of familial homicide and perverting the course of justice.
Their father, Antonio Watt, 60, was previously cleared of familial homicide and was acquitted today of perverting the course of justice.
Detectives believe Mr Gilbert died overnight between January 21 and 22 last year as a result of the abuse he had suffered.
A bag containing his headless corpse was found by two dog walkers on May 10, 2009, weighted down with a stone from the patio wall at the Watt family home.
His knee joints, forearm and elbow were missing, as well as his head.
James's bedroom, which he shared with Oldfield, has been identified as the scene where the corpse was butchered.
Mr Gilbert's severed head was not discovered until February this year in a prison-issue holdall, along with other missing body parts.
James, Robert and Richard Watt, along with their mother Smith-Dennis, Oldfield and Roberts, will return to Luton Crown Court on Monday to be sentenced.
The court heard that James Watt has 14 previous convictions, including affray and shooting someone with an airgun.
Robert Watt has two convictions, including battery and theft, while their mother has no previous convictions.
Oldfield has only one caution for affray, while Roberts has a caution for making off without payment and four convictions for the same offence.
Speaking outside court today, Mr Gilbert's mother, Rosalie White, 49 - who left court in tears when the verdicts were given - said the family were pleased with the result.
Her 31-year-old son, Chrissy, said: "It's been a traumatic time for our family and nothing will bring Michael back but let's hope this will save other people from violent behaviour."
Mrs White said the whole family had tried to help Michael, who had returned to them occasionally.
She said: "He was 26, he wasn't a baby. I did all a mother could do."
Senior investigating officer Detective Chief Inspector Jon Humphries said he had been struck by the level of inhumanity displayed by members of the Watt family.
He said: "I would say this is the worst case and most serious and long-standing series of abusive incidents that have led to someone's death that I have seen in 26 years."
The Luton Safeguarding Of Vulnerable Adults Board (Sova) today announced a serious case review in the case.
Chairman Professor Michael Preston-Shoot said: "The purpose of having a serious case review is neither to reinvestigate the case nor to apportion blame.
"The serious case review will look into the effectiveness of multi-agency and individual agency procedures to inform and improve best local practice."
He said the review was expected to begin shortly, and an executive summary would be published in due course.
Mitigating for James Watt, Andrew Jefferies QC told Judge John Bevan QC that he could not explain why the 27-year-old meted out such violence to Mr Gilbert.
He said there was a lack of pre-meditation in the murder, and Watt had admitted some responsibility with guilty pleas to some charges.
Michael Borrelli QC, representing Robert Watt, said he could be described as a vulnerable adult himself.
Mr Borrelli said: "For the first eight years of his life he grew up in an environment where to say that he was set no example is not quite right.
"He was set an appalling example and his perception of what was right and wrong must surely have been distorted."
He said Robert was on the "at-risk register", and had received as many as 72 separate visits from an education officer to help him.
"He is illiterate," Mr Borrelli said, "there's no doubt about that.
"He was suicidal, he complained of suicidal thoughts as a result of the bullying to which he himself was being subjected to at home, and all the while having the most appalling example set for him by older brothers and parents."
Geoffrey Birch, counsel for Jennifer Smith-Dennis, told the court that blame for her sons' behaviour could not be laid solely at her door.
He said she had no criminal record but had a difficult background, including an abusive first marriage.
"She was not only the victim of consistent and continued domestic violence, often receiving significant injuries, but her treatment by her former husband also produced extended problems for the three children that she bore by that first husband."
Mr Birch said that, after the 58-year-old met Antonio Watt, bearing a further four sons, she was left to deal with them alone.
"It is easy to dismiss claims that she could not handle such matters but she was just not equipped in any way," he said.
"She did not have any real support to enable her to cope with the problems, the truanting, the violence of some of her sons and the general domestic scene. It was just beyond her."
He said her fourth son, Colin Watt, who gave evidence in the trial, had admitted his mother was "fighting a losing battle".
Mr Birch said: "It is rather too simplistic to place at her door the responsibility for the mayhem and dysfunction that we have heard about in such detail over the past weeks."
The case was adjourned until 10.15am on Monday, when mitigation will continue before sentencing.
