Lamara Bell, 25, was found conscious inside the car near Stirling on 5 July but died in hospital four days later.
Her partner John Yuill, 28, died in the crash which was reported to police but not followed up.
Ms Bell's brother Martin said police investigations revealed she was able to speak to rescuers when they found her.
In a post on Facebook, Mr Bell said: "She was defo 100% conscious. She was able to tell them her name. She added 4 years on to her age and told them she was 29. She did also say she was only in the car for 20 mins.
He added: "They also said she was still moving around trying to get out the car but was trapped.
"Proud of my sis for doing so well over those three days then the following days in hospital for not giving up the fight. Brave amazing girl. She gave it her all and fought until the end."
Ms Bell, a mother of two from Falkirk, was found beside Mr Yuill in the couple's blue Renault Clio on 8 July.
The car, which had left a slip road and gone down an embankment, was reported to police on the day of the accident by a member of the public.
But the details were not entered into the police system.
It was eventually located by officers following up another call from a member of the public who had seen the vehicle.
Scottish Justice Secretary Michael Matheson has ordered a review of all police call handling and the case is also the subject of an independent investigation by the Police Investigations and Review Commissioner (Pirc).

Bryony Nierop-Reading's home went over the cliff at Happisburgh in Norfolk in the 2013 winter tidal surge.
The local council said she could not live in her caravan instead, but the Planning Inspectorate ruled she can stay until other arrangements are made.
Ms Nierop-Reading, 71, said it was a "very, very fair decision".
The former teacher moved to her 1930s bungalow on Beach Road in the erosion-hit village in 2008.
As the cliff-face disappeared, all her neighbours moved out two years later after accepting compensation from North Norfolk District Council.
Believing she would enjoy another 20 or so years of spectacular sea views, she declined.
But a few harsh winters, along with a clear-up of the ageing sea defences on the beach, left her home teetering on the edge within five years.
Following the events of the tidal surge in 2013 she moved to her caravan on her adjacent field.
However, the council issued an eviction notice saying she could not live in it long-term unless she got planning permission - which was unlikely given its proximity to the sea.
The notice was appealed and the Planning Inspectorate has now ruled the council was not wrong to issue it, but it should increase its grace period from six to 12 months to give Ms Nierop-Reading more time to make permanent living arrangements.
She now has until the end of next year to vacate.
She said the first option was for her daughter and son-in-law, who are tenants in her Witton Heath home three miles inland, to buy it from her, giving her money for another house in Happisburgh.
But failing that she said she had "a number of other options".
Ms Nierop-Reading said she believed common sense had prevailed, but "only because the inspectorate forced it".
"I am beginning to come to terms with it all and I can now think about the bungalow and some very happy times we had there," she said.
Judy Oliver, North Norfolk District Council's cabinet member for legal services, said: "The planning inspector dismissed the appeal on three of four grounds.
"On the fourth, the period of compliance was extended from six to 12 months. It's important to note that although the enforcement notice has been varied in this way, it has also been upheld.
"We thank the inspector for his decision."
A young mother who lay trapped in a car for three days after it crashed on the M9 was trying to escape when rescuers arrived, her brother has said.
[NEXT_CONCEPT]
A woman who lost her bungalow to the sea and faced being evicted from a temporary home on her land has been allowed to stay put for another year.