The ex-fiancee of Steven Avery, the subject of the Netflix documentary series Making a Murderer, has reportedly expressed fears that her former partner could be released from prison after a judge overturned the murder conviction of his nephew, Brendan Dassey.

Avery and Dassey were both found guilty of the 2005 murder of Teresa Halbach of Wisconsin in separate trials, chronicled in the hugely popular documentary series released last year.

On Friday, US federal magistrate William Duffin handed down the ruling in Dassey’s case on the grounds that he was coerced into making his original confession. In so doing, the judge called into question the conduct of Dassey’s attorney, Len Kachinsky, as well as those of investigators who he said had elicited an “involuntary” confession from the then 16-year-old, who has learning difficulties.

According to the entertainment news site TMZ, Avery’s former partner Jodi Stachowski said after the judgment that, while she had great sympathy for Dassey and believed the state of Wisconsin had a duty of care towards him “because they wilfully destroyed his life”, she feared his potential freedom might put pressure on the courts to release Avery, “who she firmly believes is guilty of raping and killing” Halbach.

TMZ added that, in contrast, Avery’s brother Chuck said that he would not celebrate “until the other person wrongly convicted is also freed: his brother”.

The reports come amid mounting questions over the impact Dassey’s case will have on that of his uncle, who is also serving life in prison. Avery’s legal team said in a statement that they were “thrilled” for Dassey that his conviction had been overturned. “We fully expected this outcome from an unbiased court that carefully examined his confession,” they said.

Avery’s attorney, Kathleen Zellner, who took up the appeal case in January, added: “Steven Avery is so happy for Brendan. We know when an unbiased court reviews all of the new evidence we have, Steven will have his conviction overturned as well.”

Jerry Buting, Avery’s former lawyer, who featured prominently in Making a Murderer, also tweeted that “justice finally strikes”, adding: “State of Wisconsin should accept federal court decision and drop case against Brendan Dassey. Avery is next. Time to go after real killer.”

Steven Drizin, a professor of law at Northwestern University, who pushed to have Dassey’s conviction overturned, told the BBC’s Today programme yesterday: “I thought that this was a confession that was the result of police coercion. These detectives had taken advantage of a young man who had severe learning difficulties. The only facts in the confession had been fed to him by the investigators.”

Halbach, a photographer for Auto Trader magazine, went missing on Halloween in 2005 after photographing a vehicle at Avery’s salvage yard. Her charred remains were discovered in a burn pit on the yard 10 days later, along with her Toyota RAV4, mobile phone and car keys. The Netflix documentary brought international attention to the subsequent murder case by exploring issues and procedures in the Manitowoc County sheriff’s department investigation of Avery and Dassey.

It also cast doubt on the legal process to convict both men, which led to a significant public backlash against the state of Wisconsin. Following the show’s airing, a petition to investigate and pardon the Averys “and punish the corrupt officials who railroaded these innocent men” was submitted to the White House with more than 120,000 signatures. (A White House spokesperson later said that, since Avery and Dassey were both state prisoners, the president could not pardon them.)

Meanwhile, Kachinsky, who was removed from Dassey’s case and later de-certified from the public defender’s office, reported receiving hate mail from Dassey’s supporters. On Friday, Duffin accused the attorney of spending more time talking to the press about the high-profile case than actually communicating with his own client. In his first three weeks as Dassey’s attorney, Kachinsky spent 10 hours speaking to reporters and one hour with Dassey, according to Duffin.

If prosecutors do not refile charges within 90 days, Dassey will walk free. While a second series of Making a Murderer is currently in the making, the new ruling is understood not to have been captured on camera.