In the scam, cheap homes in Hull were bought and then remortgaged for an inflated value.
The original prices were deliberately hidden from the lenders that provided the loans, according to the prosecution.
In one case, a terrace house in west Hull was bought for £20,000 and remortgaged on the same day with a loan based on a stated value of £91,000.
Prosecution barrister Guy Gozem QC at the start of the trial labelled the defendants “vultures” who had exploited the cheap purchase price of the properties, according to the Hull Daily Mail.
He reportedly said: “It is the prosecution’s case that the truth about these bargain prices was kept hidden from the lenders because if they had known about it they would not have lent on the same basis.
“In that sense, a bargain price was an inconvenient truth for these defendants. They needed to find a way of hiding the prices from the lenders.”
The brothers, along with Darren’s partner Julie Crane, were found guilty of 10 charges of conspiracy to commit fraud by false representation.
The Goulbournes and Crane had denied the joint charges.
Solicitor Christopher Jones was found guilty on nine charges on conspiracy to commit fraud but was cleared on a charge relating to one property.
Solicitor Katie Baxter was found not guilty on four charges of conspiracy, with the jury unable to reach a majority decision on a further six charges against her.
Andrew Wormstone pleaded guilty to three charges before the trial started.
Darren Goulbourne and Crane were cleared on a separate conspiracy charge involving another lender.
Sentencing in the case is expected at the end of April and custodial sentences are a possibility.
Judge David Tremberg reportedly said: “All options are open. I am not tying myself to anything at the moment.”