Michelle McGinty said the report from the inquiry into the C. diff deaths at the Vale of Leven Hospital in West Dunbartonshire had been put back for the fourth time.
Health Secretary Alex Neil said Lord MacLean, who chaired the public inquiry, required extra time to fully consider all of the evidence.
A total of 55 patients at the hospital were struck down by the bug between December 2007 and June 2008, and 18 people died.
C.diff was blamed for nine of the deaths and was said to be a contributory factor in the other nine.
The inquiry, which was ordered by then Health Secretary Nicola Sturgeon, was originally due to report by the end of May 2011.
Mrs McGinty, whose mother- in-law Sarah McGinty was one of those who died in the outbreak, campaigned with the C.diff Families Justice Group for the inquiry to be set up.
She said: "We're coming up for the sixth anniversary of the first deaths, and the families need closure so they can move on with their lives.
"It is completely unacceptable that after six years and four delays we are still no further forward on knowing what happened at the Vale of Leven."
Jackie Baillie, Labour MSP for Dumbarton, also criticised the delays, and said: "It has now been four years since the inquiry was set up and it is unthinkable that families should have to wait even longer for the answers they deserve.
"Of course, it is important that the inquiry has the time to consider all the evidence but this is now the fourth time that the report has been delayed.
"It is important that all of Scotland's hospitals learn the lessons from the Vale, and this delay does not help."
Ms Baillie had submitted a question to the Scottish Government about when ministers expected to receive Lord MacLean's report.
In his answer, Mr Neil said: "I recently received a progress report from Lord MacLean, chairman, Vale of Leven Hospital inquiry, in which he indicated that he will now endeavour to publish his final report by March 31 2014."
The Health Secretary said he had been assured "good progress is being made" but Lord MacLean said additional time was needed "to fully consider all evidence to ensure a robust report".
Mr Neil added: "The timing of the report is entirely a matter for Lord MacLean."
A Scottish Government spokeswoman said: "The Vale of Leven Hospital inquiry is an independent inquiry.
"The procedure and conduct of an inquiry is determined by the inquiry's chairman in accordance with his/her duties under the Inquiries Act 2005.
"The Cabinet Secretary for Health and Wellbeing has granted an extension to the Vale of Leven Hospital inquiry, as requested by Lord MacLean, chairman of the inquiry. Whilst the inquiry continues to make good progress, Lord MacLean requires additional time to consider all evidence to ensure a robust and meaningful report.
"The Health Secretary has therefore agreed to a revised reporting date and Lord MacLean will publish his final report by March 31 2014."