Amirah Droudis, 37, will spend at least 33 years behind bars for killing the woman -who cannot be identified - in 2013.
Droudis's boyfriend, Man Haron Monis, took 18 people hostage in a Lindt cafe in central Sydney in 2014.
The 16-hour siege ended with the deaths of Monis and two hostages when police stormed the building.
Monis had been charged with being an accessory to his ex-wife's killing, and was on bail at the time of the siege.
The Supreme Court of New South Wales heard that Monis planned the 2013 murder and Droudis carried it out.
The victim, identified by the pseudonym Helen Lee, was stabbed 18 times before being doused in petrol and set alight outside an apartment in western Sydney.
After the trial, Justice Peter Johnson ruled that Monis recruited Droudis to murder his ex-wife.
"The offender uncritically adopted and espoused Monis's foul beliefs and acted in public support of him in public protests," he said in his sentencing remarks on Wednesday.
The judge described Monis as "an evil man" whose death was "a result of his own criminal and murderous acts".
"No-one mourns his passing and many have been left to grapple the consequences of his destructive acts," he said.
The judge acknowledged claims that Droudis had been repeatedly assaulted by Monis.
Droudis was sentenced to a maximum 44 years in jail with a non-parole period of 33 years.
Detective Inspector Jason Dickinson, who worked on the case, said he was satisfied with the sentence handed to Droudis.
"This was a brutal and callous crime and I think the sentence today has reflected that brutality," he told the Australian Broadcasting Corp.
The victim's family made a statement outside court, thanking the judge, prosecutors and police.
"Today we are very happy that justice has been served to our only daughter," the statement said.
The findings of an inquest into the cafe siege are due to be handed down this year.
How the Sydney siege unfolded
The girlfriend of a man behind a deadly siege in a Sydney cafe has been jailed for murdering his ex-wife.