John Gimbert, a former constable in Staffordshire for 30 years, abused his position as power of attorney over Janette Trim’s financial affairs.
Miss Trim, who is in her 60s, suffers from severe learning disabilities.
Mr Gimbert asset stripped his cousin, selling her bungalow for just £1 to his son David. It was worth between £50,000 and £80,000.
David, a serving sergeant based at Staffordshire Police HQ in Stafford, received a six-month suspended prison sentence for his role in the plot.
The pair faced a retrial over a charge of conspiracy to defraud after jurors failed to reach verdicts at an original trial in February.
They were found guilty by a jury at Birmingham Crown Court last month.
John Gimbert, 65, of Westwood Park Avenue, Leek, was found guilty of four counts of theft between November 2002 and October 2004 in the original trial. He used the money to help his three children buy homes.
Sentencing John Gimbert, Recorder Benjamin Nicholls said: “You grossly abused a position of trust over your cousin, who you knew to be extremely vulnerable.
“You created a labyrinth of bank accounts to conceal what you were doing, which was to allow your children to get on the housing ladder.”
For the fraud charge he was jailed for three-and-a-half years, and he received four separate sentences ranging from 12 months to three-and-a-half years for theft. They will run concurrently.
The court was told John Gimbert raided Miss Trim’s inheritance, following the death of her father, by giving large cash payments to his two sons and daughter.
He bought three new Chrysler cars worth thousands of pounds with money taken from her accounts. He and his wife would also use a ‘Motability’ car leased using her benefit money for their own purposes.
The ruse was exposed after staff at Staffordshire County Council became suspicious about John’s spending for his cousin such as for a new computer she would not have been able to use.
The court was told John Gimbert paid some of the money back – but the prosecution said this only came after concerns were raised by social services and carers.
Sentencing David Gimbert to three-and-half years suspended for 12 months, Recorder Nicholls said: “It is frankly a tragedy that you stand before me in crown court. You have lost your career, your reputation, and your relationship with your father.
“You knew your aunt, as you called her, was vulnerable. As a police officer you should have been able to stand up to your father.”
David Gimbert, 37, of High View, Mow Copp, was given a ‘glowing reference’ by a superintendent at Staffordshire Police.
The court heard he had been commended by former Labour Home Secretary John Reid for crime reduction.
His barrister, Mr Nick Gibb, said his client was ‘completely devastated’ but maintained his innocence and ‘stood at the mercy of court’.