Christina Sethi targeted three victims, one male and two female, at a care home in Devon, south-west England, between January 2014 and May this year.
She also filmed the assaults on her mobile phone before sharing them with her boyfriend.
Plymouth Crown Court had sentenced her to 10 years after a trial in August. However, after a reference to the Court of Appeal, three senior judges decided to hike the sentence.
Sethi deliberately targeted victims suffering from dementia, the oldest of whom was a 101-year-old female.
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One of her victims has died since the incident. Sethi had admitted three counts of sexual assault and two of assault by penetration in July.
During the sentencing, judge Richard Stead had said she had committed "horrific abuse of three vulnerable and elderly victims who were in a care home under your care."
"You humiliated three elderly people who trusted you," he said.
Devon and Cornwall Police described Sethi's actions as "incomprehensible."
"She committed these atrocious acts on the people she had been trusted to care for and targeted their vulnerabilities in order to satisfy her own sexual deviance," Detective Inspector Ed Wright said.
The abuse came to light when a man who knew Sethi bought a computer from her and found deleted films of the sexual assaults which he reported to police.
Thevideo files showed her sexually abusing two elderly female residents, one of whom was recorded helplessly asking, "What are you doing to me?".
A 32-year-old man who was arrested on suspicion of conspiracy to commit sexual assault has since been released without charge.