The decision released by a judge Tuesday means that Shayanna Jenkins could be compelled to testify or face time behind bars.
Prosecutors filed the petition last month, and Superior Court Judge Susan Garsh held a closed hearing at that time but did not immediately release a decision. All details of the petition were sealed at the time it was filed. The court said it was because the petition contains grand jury testimony.
It wasn't clear when or if Jenkins would be called to testify.
Hernandez is charged with the June 2013 killing of Odin Lloyd, who was dating Jenkins' sister.
Jenkins has been seen often in court during the trial, sitting behind Hernandez to support him. The two have been heard whispering "I love you" to each other and occasionally joking together. Jenkins last week also shared notes with Hernandez's defense team after her sister testified against Hernandez.
She still lives in the couple's North Attleborough mansion with their 2-year-old daughter.
Prosecutors have said Jenkins lied to a grand jury dozens of times, including when she said she didn't know what she did with a box she took from the basement of their home that prosecutors say contained possible evidence. She has pleaded not guilty to perjury.
The gun used to kill Lloyd has never been found.
Hernandez's trial is expected to resume Wednesday after a two-day snowstorm delay.
Hernandez's Fiancee Granted Immunity
ESPN legal analyst Lester Munson discusses the news that Shayanna Jenkins, the fiance of Aaron Hernandez and mother of his daughter, has been granted immunity in Hernandez's murder trial.