Harry Dunn’s twin brother has told Prime Minister Boris Johnson “we have had enough of the lies”.
Niall Dunn, 20, said he and his parents have been “going through hell” since his brother’s death in a crash outside RAF Croughton in Northamptonshire.
He wrote to Mr Johnson, saying he was “sick and tired” of seeing his parents suffer and urged him to intervene.
Suspect Anne Sacoolas left for the United States after the crash in August, citing diplomatic immunity.
Niall said he was confident the prime minister – who has not publicly commented on his letter – would respond personally.
“I’ve had enough of the lies, the deceit. They think they can just walk all over us and get away with it,” he said.
“My parents have been and are going through hell. So am I. We all are.
“I just don’t understand how they think they can get away with it.”
In the letter, he urged Mr Johnson to take up the case as his family believes the Foreign Office has “made a mess” of the case and Foreign Secretary Dominic Raab had “lost control of this scandal”.
Part of his letter reads: “It was bad enough losing Harry. But watching [my parents] go through this torture is just awful. It’s just cruel.”
“Please get involved in our case… We are not the dirt at the bottom of the government’s shoes. We are UK citizens and we have the right to know the truth.”
Mrs Sacoolas, the wife of an intelligence official based at RAF Croughton, was charged with causing death by dangerous driving in December.
However, an extradition request for her to return to the UK, submitted by the Home Office, was rejected by US authorities in January.