Now they face lengthy prison terms. But neither showed any emotion as the jury foreman returned the verdict at Leeds Crown Court, London on Thursday.
The mother stood staring straight forward in the stone-coloured jacket she has worn throughout the proceedings and with her red hair hanging untied over her shoulders. Her brother Donovan stood a few feet away from her, separated by a court security officer.
There was silence from the rest of the court following the verdict after the judge warned the public gallery against over-emotional responses.
The jury of seven men and five women took around six hours to find Matthews and Donovan unanimously guilty of all charges.
They remained emotionless as Mr Justice McCombe warned they face a 'substantial custodial sentence'.
He said earlier that he would adjourn sentencing for reports.
No application was made for bail and the two defendants were led away.
Speaking outside the court, Detective Superintendent Andy Brennan, who led the investigation, described Matthews as 'pure evil'.
He said she deceived those closest to her from the moment Shannon was kidnapped.
'They had no reason to believe Shannon had been taken as part of an elaborate scam or hoax,' he said.
'It is difficult to understand what type of woman would subject her own daughter to such a wicked and evil crime.
'Karen Matthews is a manipulative individual who has demonstrated a remarkable ability to lie.
'The one person that Shannon should have been able to rely on more than any other person is her own mother.
'She has been totally betrayed by her own mother. Karen's interest and motivation throughout this has not been for Shannon or anybody else, it's been for her and getting her hands on £50,000.
'It really does beggar belief that a mother would put her own child in this position.'
Outside court, friends and neighbours of Matthews spoke of their feelings following the conviction.
Petra Jamieson said she was shocked that Matthews showed no emotion as the verdicts were delivered.
She said: 'She didn't seem bothered. All us lot have been here all the way through it, but she didn't seem to give a s**t
'I don't think it has hit home yet. Maybe it will when she comes to be sentenced.
"I can't believe she showed no emotion.'