Kramonthas Thanathornkositcheera, 62, will serve the maximum of 50 years in jail allowed under Thai penal law after pleading guilty to scores of charges including 29 counts of royal defamation.
The Thai monarchy is protected by one of the world's toughest lese majeste laws.
She was accused of swindling four people, who owned a printing factory, out of around $145,000 between 2010 and 2014.
She made promises that donations would be rewarded with palace favours in a country where the monarchy holds great prestige and powers of patronage.
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To back up her elaborate claims she falsified royal household documents and pretended palace officials attended a birthday bash she threw for herself.
"The defendant was given a total 150 years in jail," said the prosecutor -- who was not named in court documents seen by AFP.
The prosecutor explained she will serve the legal maximum of 50 years after pleading guilty.
Kramonthas is half-sister to Monta Yokrattanakan, better known as 'Ying Kai', who is awaiting trial over royal defamation and human trafficking charges.
The monarchy is a highly sensitive issue in Thailand, with the law and social opprobrium swiftly brought down on anyone deemed critical of the institution.
All media based in Thailand must self-censor to avoid falling foul of the broadly worded law.
The death of Thailand's revered King Bhumibol Adulyadej on October 13 at the age of 88 plunged the country into intense mourning.
Bhumibol's son Crown Prince Maha Vajiralongkorn will take the throne.
The country's ruling junta has vowed to double down on criticism of the monarchy.
A famous Thai fortune teller was arrested in October last year and charged with improperly using his connections with the monarchy to make money.