Sacoolas was driving on the wrong side of the road when she crashed her Volvo and killed the 19-year-old motorcyclist in August 2019.
The 45-year-old pleaded guilty to causing death by careless driving in October, where sentencing judge Mrs Justice Cheema-Grubb urged her to return to Britain to face justice.
But it has emerged the US Government has advised Harry Dunn's killer not to attend her sentencing hearing on Thursday - with a renewed application for her to appear via video-link granted.
The Dunn family spokesman, Radd Seiger, said the teenager's parents were "horrified" to learn the US Government is "actively interfering in our criminal justice system".
He said: "Harry's family are victims of a serious crime and they have been kept in the dark completely about what is to come at Thursday's hearing since Mrs Sacoolas's guilty plea on October 20.
"We are horrified to learn that the United States Government is now actively interfering in our criminal justice system.
"Their ongoing cruel treatment of Harry's parents is nothing short of inhumane and it continues to take a heavy toll on their mental health.
"If there is a genuine reason why Mrs Sacoolas should not appear in court on Thursday, as directed by the judge, then the parents would happily accept that.
"But on the face of it, it appears that this is nothing short of a cowardly act on the part of an oppressor.
"I have today asked for an urgent meeting with the Foreign Secretary James Cleverly to understand what action the British Government intends to take in response."
Sacoolas was driving on the wrong side of the road near RAF Croughton, a US air base in Northamptonshire, when she crashed her Volvo and killed motorcyclist Mr Dunn.
She had diplomatic immunity asserted on her behalf by the US administration following the collision, and left the UK 19 days later. It led to a lengthy battle for justice by Mr Dunn’s family.
Sacoolas has attended both previous court hearings via video-link from her lawyer's offices in Washington DC.
Confirming she would not have to attend in person at the sentencing on Thursday, a court spokesman said: “The application made jointly by the prosecution and defence for Mrs Sacoolas to participate and be sentenced by live link, has been renewed.
“The defence have supplied material in support of the application including evidence that Mrs Sacoolas’ government employer has advised her not to attend in person. The judge has granted the application.”