Housebound Maureen Brown, who suffered from ME, had lost her beloved husband and carer Ian to cancer only weeks previously.
Maureen, who was the sister of Inverkeithing and Dalgety Bay councillor Alice McGarry and presiding officer of the Scottish Parliament Tricia Marwick, was being cared for at her Husband Place home by son Graeme.
Next-door neighbour Jackie Todd became worried when he returned from working nightshift at 8am to find a faint burning smell in his house.
After failing to receive an answer on Maureen's phone he knocked frantically on the door of her mid-terraced house but again received no reply.
Jackie said, "At first I thought it was an electrical fault in our house so I went and checked everything, even going up the loft.
"I tried to phone next door but the phone was engaged all the time.
"I knocked on the door to see if everything was okay then lifted the flap of the letter box and the smell just hit me."
Jackie called on fellow neighbour Alan Whiting, an ex-paramedic, to help.
Alan recalled, "The windows appeared to be blackened and Jack said there appeared to be a funny smell.
"There was a faint alarm going off. It's a tragedy."
Two appliances from Fife Fire and Rescue rushed to the property to find that the flames had been extinguished.
However, the home was heavily smoke-logged and Mrs Brown was found within the house.
Alice McGarry, who lost her own son, Ross, in a house fire in 2004, paid tribute to her sister and her family, with Maureen's daughter, Lorraine, having to fly back from holiday.
She said, "Maureen had ME for 20 years and was housebound. We all knew she wasn't very well but what has happened has come completely out of the blue.
"She was the oldest in a family of seven. We all looked up to her as head of the family. In her day she was full of life and vigour.
"Ian was also a wonderful person who showed great dedication in looking after her and it was absolutely awful what happened to him.
"Graeme was there three or four nights a week. He was full-time carer for his dad when he was dying and looked after his mum when he passed away."
Ian, who used to work for FMC and at Rosyth Dockyard, had retired five years ago to care full-time for his wife. However, tragedy struck the family earlier this year just months after Ian reached 65.
Jackie and wife Isabel had lived next door to the couple for more than 30 years.
Isabel said, "Ian had looked after Maureen for about 20 years. He just got his state pension in January and in March he was diagnosed with cancer then, as quick as that, he died only five weeks ago. That was such a shock.
"Graeme was really good and still stayed with his mum even after the funeral but he's got kids of his own.
"He kept coming through every day to make sure she was fed and everything was alright.
"He was in there last night (Sunday) and was going to come in this morning but had a family emergency or he would have been through like normal. He didn't come through and this has happened."
Jackie added, "They were lovely people, very family-orientated. I don't think you'll ever find anyone who could say a bad word against them.
"Ian never had a bad word to say about anybody.
"With him being retired and me being semi-retired every day we were going out there and putting the world to rights across our garden fence.
"It was a great friendship and they will both be sadly missed."
Detective Inspector David McLaren, who is leading the investigation, said, "At this early stage, indications are that there are no suspicious circumstances, although we will continue to keep an open mind.
"A report has been submitted to the procurator fiscal at Dunfermline outlining the circumstances of her death."