The family, who were living in Dersingham at the time, took over the woman's finances after her father died leaving her £120,000 in the form of a family home which she was unable to manage.
They bought the woman a £43,000 mobile home then spent most of the rest of the money on themselves, paying for holidays to Australia and other luxuries.
The matter came to light after the victim was found living in debt and a neighbour contacted police.
A two-year police investigation discovered that £70,000 had been stolen from her and on Friday, at Norwich Crown Court, retired husband and wife, Stephen and Denesse Hume, both aged 64, of Great Shelford, near Cambridge, admitted offences of theft and money laundering. Their daughter's estranged husband, Giovanni Marasca (43), of Cambridge, had pleaded guilty to two charges of money laundering at an earlier hearing in the same court.
Marasca was not in court on Friday but all three defendants will now be sentenced provisionally on November 21. They are all on bail.
'LAUNDERing'
Denesse Hume admitted 14 offences of theft from the Hunstanton woman between May 13, 2005, and September 6, 2005, involving more than £65,000.
She also admitted 14 offences of money laundering.
Stephen Hume admitted five offences of money laundering, and now the couple face a confiscation order hearing on February 6 next year. Norfolk police's Adult Abuse Investigation Unit has been investigating the case, involving more than 50 witnesses and hundreds of financial exhibits.
Acting Detective Sergeant Stephen Phillips, who headed the investigation, said: "It's a particularly wicked crime to take advantage of someone like our vulnerable victim.
"In effect, they took her money and then left her to get on with it – it's a crime of greed."
He said the Humes were introduced to the Hunstanton woman by a member of her family living in that area in late 2004.
"The victim had been looked after by family members throughout her life and when her father died he left an estate of £120,000 in the form of a family home which she was unable to manage," he said.
After befriending her, the Humes and Marasca took over the woman's finances and in May 2005, sold the house, which was supposed to support her into her old age.
"They purchased the victim a mobile home for about £43,000, including fixtures and fittings like a TV and microwave, and put her in a mobile home park," he said.
"Then, within 14 weeks, they stole all the remaining money, putting it in personal accounts and spending it on themselves paying for trips to Australia, other holidays abroad, credit card bills and other luxuries.
"By the time we were alerted to this in October 2006, the victim was in financial debt. A major investigation was launched and Norfolk County Council Adult Social Services immediately stepped in to support the woman."
Tim O'Mullan, head of community teams, Norfolk County Council Learning Difficulties Service, said: "This was an outright breach of a vulnerable person's trust and it is only right that the perpetrators are going to be brought to task.
"We have worked closely with Norfolk Police during the course of the investigation and have helped to provide support to the victim.
"I hope bringing this case sends out a clear message that Norfolk County Council Adult Social Services and Norfolk police have very good arrangements in place that ensure these cases are acted upon and that we will not accept this kind of exploitation."