The US State Department has labelled the British Government's official request for Anne Sacoolas to be sent back to the UK on Friday (January 10) as 'an abuse' of the extradition treaty.
Family spokesman Radd Seiger urged people not to listen as he is adamant she will return having fled to the US claiming diplomatic immunity following the crash near Croughton in August.
"Friday was clearly a monumental day for us with that extradition request being sent across," he said in a video on Twitter yesterday (Sunday).
"I certainly expect the good people at the State Department and the Department of Justice to deal with it in accordance with the extradition treaty and be the good professionals that I know they are.
"Please disregard all the nonsense coming out of the State Department at the moment - you may disregard that as Anne Sacoolas will be coming back to the UK to face the justice system."
Harry, 19, died in hospital after the crash with a car while riding his motorcycle on the B4031 between RAF Croughton and the village of Croughton on August 27.
Mrs Sacoolas was charged with causing death by dangerous driving in December after months of campaigning by Harry's family which has seen their plight make headlines around the world.
Following the Home Office extradition application being formally sent, a US State Department spokeswoman said: "It is the position of the US Government that a request to extradite an individual under these circumstances would be an abuse.
"The use of an extradition treaty to attempt to return the spouse of a former diplomat by force would establish an extraordinarily troubling precedent."
The Home Office said the decision is now in the US Government's hands but it is unclear when a decision will be made.
A lawyer will now decide whether it falls under the dual-criminality treaty, where the alleged offence is a crime in both countries and carries a prison sentence of at least a year.
The maximum punishment for causing death by dangerous driving is 14 years in jail but a judge would take lots of different factors into account before deciding on the sentence, should a defendant be convicted.
The US has always argued that Mrs Sacoolas is entitled to diplomatic immunity - Northamptonshire Police has argued it no longer applies because she is not in the country anymore.
While Foreign Secretary Dominic Raab has said there is an 'anomaly' with how diplomatic immunity relates to spouses at RAF Croughton.
In a tweet, Mr Seiger said it was 'not cool' for the American administration to pre-judge the application but has faith the US will not go against the extradition treaty.