The Minister for Justice, Mr McDowell, has ruled out a public inquiry into the unsolved murders in 1997 of two women in Grangegorman, Dublin.
Labour criticised the Minister after he said in a response to a parliamentary question that he was "not at present satisfied" that a public inquiry was required.
Ms Sylvia Shiels (58) and Ms Mary Callinan (61) were stabbed to death in their sheltered housing accommodation at St Brendan's psychiatric hospital in Dublin in March 1997.
Ms Shiels's sister, Ms Stella Nolan, said the Minister's refusal to grant a public inquiry was very hard to bear as no-one was ever charged in connection with the case.
"It beggars belief that the Minster should now say that a public inquiry is not even required," she said in a statement last night.
Although two people made separate confessions to the murders within months of each other, she said the Garda and DPP had never explained why there was no prosecution in the case.
"If this sequence of events does not require a public inquiry, I would like to know just how serious matters must be before the Minister for Justice would be convinced that a public inquiry is required?"
Mr McDowell said in his response to the parliamentary question that the decision followed legal advice from the Attorney General, Mr Rory Brady.
Labour's justice spokesman, Mr Joe Costello, said it had never been made clear why a prosecution was not taken against Mark Nash, who confessed to the attacks when he was picked up for the double murder of a couple in Roscommon.
Nash later withdrew his statement and is in Arbour Hill prison for the murder of Carl and Catherine Doyle.
Dean Lyons, a homeless drug addict, had earlier confessed to the Grangegorman killings, and spent time in custody. Questions were raised about the confession and the charges against him were dropped. He moved to England, where he died in 2000.
