Lamara Bell was critically injured in the crash on July 5, but she and her partner John Yuill, 28, were only discovered by officers on July 8.
Yuill had already died when police reached the accident spot. Bell had been in a medically-induced coma since then.
Bell, who was a mother, was being treated at the Queen Elizabeth University Hospital in Glasgow having suffered kidney damage from dehydration from lying in the wreckage for so long. She died early today, the BBC reported.
"My sister just passed away," he said.
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A statement on behalf of the Bell family said: "Sadly, our daughter has passed away. We now request that the media respect our privacy to grieve for Lamara at this very difficult time."
A statement on behalf of the Yuill family said: "The family of John Yuill would like to say that their prayers and thoughts are with Lamara and her family. We are devastated by the sad news this morning."
The Police Investigations and Review Commissioner is reviewing the handling of the incident after it emerged that police had received a call about the crash on Sunday, but the information had not been entered into police systems.
"We will continue to co-operate fully with the Police Investigations and Review Commissioner as they undertake their investigation into the circumstances of this tragic incident."
Last week Stephen had apologised to the couple's families for the "individual failure in our service".
The couple had been reported missing to police after last being seen in the company of friends in the Loch Earn area of Stirlingshire in a blue Clio in the early hours of Sunday.
Stephen said a member of the public had called the 101 non-emergency number on Sunday morning after seeing the car down the embankment near the Bannockburn slip road.
The call had been taken by an "experienced officer", who has since remained on duty. However, "for reasons yet to be established" this was never entered into systems or sent out to operational teams in the area.
"That we failed both families involved is without doubt," the chief constable had said.