- Video report by ITV News Correspondent Stacey Foster
Speaking after a meeting with Business Secretary Andrea Leadsom, Charlotte Charles said: "She will come back, I don't care how long it takes, our fight for justice for Harry will not waiver our determination if anything, it is stronger now than it was before, if it's even possible."
She added: "She has to come back, I don't even know how she can live with herself and carry on with her life and drive."
"I don't understand that, she must be made of different stuff. But she has to come back one way or another," she said.
Her comments comes after the US secretary of state refused an extradition request, for the suspect charged with causing the death of the teenage motorcyclist by dangerous driving.
Ms Charles continued: "I'll make it my life's work if I have to. We have to, we all will.
"They can beat me down as much as they want to. Every hurdle they've put in the way so far, whether it's been here or there, we've kept the fight and been able to get over those hurdles."
Dominic Raab earlier said the Foreign Office believes Anne Sacoolas should return to the UK and branded the US's decision as a "denial of justice".
In a statement he added the "UK would have acted differently".
He said: "I called the US ambassador earlier to express the Government’s disappointment about this decision."
He continued: "We feel this amounts to a denial of justice, and we believe Anne Sacoolas should return to the UK."
The Home Office submitted an extradition request for Sacoolas earlier this month, after she was charged with causing the 19-year-old’s death by dangerous driving in December.
Anne Sacoolas, the wife of a US intelligence official, claimed diplomatic immunity after the collision and was able to return to her home country, sparking an international controversy.
Mr Dunn died after his motorbike crashed into a car outside RAF Croughton in Northamptonshire on August 27 last year.