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.
Police are searching the side of a motorway where two people were left in a crashed car for three days before being discovered.
John Yuill, 28, was killed in the smash near Junction 9 of the M9 near Stirling, Scotland. His girlfriend Lamara Bell, 25, is in a critical condition.
Their families have slammed police for allegedly ignoring an initial 999 call reporting the crash, and an investigation has been launched into why the call was not followed up at the time.
A manhunt had been launched earlier this week when the couple were reported missing.
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.