Lamara Bell, 25, who lay in a crashed car off the M9 near Stirling for three days, has died, her brother Martin Bell said.
Her partner John Yuill, 28, died in the accident. Police have apologised to their families for the delay in providing help.
Scotland's Justice Secretary Michael Matheson has now launched an urgent review into how the police handle emergency calls.
Scottish Liberal Democrat leader Willie Rennie has said "serious questions must be answered" in the wake of the M9 car crash.
"I have been contacted by constituents and serving police officers who are horrified by this tragedy," he said.
"They want a thorough investigation into the serious issues raised by the case and whether they are connected to the wider problems with the operation of Police Scotland.
"Everyone agrees that a three-day delay between the initial call to the control room and finding the couple is unacceptable and we need to establish why this happened."
He added it was right that the Police Investigations and Review Commissioner (PIRC) is investigating the matter but said a a wider independent investigation may be needed.
Scottish police failed to react to a call over a fatal car crash because the information was not logged on their systems.
An investigation has been launched over fears a couple involved in a fatal road crash may have lain in their car for days.