Defence counsel Andrew Dawson said his client's offending was triggered by gambling problems and stressed she had serious health problems which culminated in her having a stroke last year.
Between 2007 and 2016, McMahon stole $65,110 from her father, Arthur Lester, and her brother, Brian Lester.
The latter was hit hardest, being ripped off by $35,810.
It was fraud but it was not complex.
As caregiver to the men - aged 84 and 69 - McMahon had authority to use their bank accounts to cover their day-to-day costs and bills.
While she made sure they were covered, she also helped herself.But McMahon showed "cruel premeditation" by coercing her brother, who struggled to read and write, into allowing her access to a trust account, the judge said.
"A particularly cruel aspect of this is to go as far as getting into a trust fund, which is locked up, and to get your brother's consent to do that when he wouldn't have known what he was doing is so much worse than accessing bank accounts when there's cashflow coming in," Judge Crosbie said.
Mr Dawson said his client had been left alone by the other siblings to care for the two men.
"The burden fell on her but ultimately wore her down over a period of years," he said.
"She became very stressed and anxious and resentful to a degree."
Gambling allowed her some respite from that strain, Mr Dawson said.
But her siblings rubbished her explanation in their statements to the court.
They said she had plenty of help in looking after the vulnerable duo.
"What you've done completely ripped the family apart," the judge said.
"I expect if your father knew about the fraud there wouldn't be any [inheritance] for you ... the will would probably have been changed," Judge Crosbie said.
Mr Dawson said his client had voluntarily repaid her inheritance of $32,000 and was willing to repay the $65,000 at $100 a week if she kept her job.
He believed a sentence of community detention - a number of months on a curfew - was a suitable penalty. Judge Crosbie did not. He referred specifically to McMahon's previous appearance before the court in 2006 when she admitted seven charges of dishonestly using her mother-in-law's bankcard.
She was sentenced to 120 hours' community work and ordered to pay $400 reparation on that occasion.
"Given your previous history, given the amount stolen was a large sum, given you have an effective inability to pay that back ... I am of the view that a sentence of imprisonment is required," the judge said.
He jailed McMahon for two years.
No order for reparation was made.
It is understood the 52-year-old will appeal the sentence.
