Harry Dunn's family has been given permission to go ahead with a civil claim against the teenager's alleged killer and her husband.
The ruling was made by a judge in the Alexandria district court in the US state of Virginia on Wednesday, allowing the civil claim against Mr and Mrs Sacoolas and a claim of "vicarious liability" to be brought against the suspect's husband.
The law of vicarious liability in Virginia means that Mr Sacoolas could be liable for the teenager's death by allowing his wife to use the car which killed him.
The Dunn family's spokesman Radd Seiger told the PA news agency: "Harry's family are very pleased at the ruling today and that their claims have been allowed to proceed in full.
"They are also pleased that Mr Sacoolas will also have to be deposed so that they can learn the full account of what happened on the night Harry died.
"The Sacoolases and their insurers, the United Services Automobile Association, have sought to deny Harry's family justice at absolutely every turn along the way.
"Common sense has prevailed tonight."
Mr Seiger continued: "The Dunn family are entitled to pursue their legal rights to the fullest extent possible, ultimately to be determined by the court at trial.
"It is time to get on with delivering justice to this family and to put these petty attempts to deprive them of their rights behind us.
"The USAA have done themselves a huge disservice in their handling of the claims to date."
Anne Sacoolas was leaving RAF Croughton in Northamptonshire in August 2019 when she was involved in a crash with 19-year-old Harry, who was on his motorbike.
She had been driving on the wrong side of the road but claimed diplomatic immunity and returned to the US with her family.
Despite being charged with causing Harry's death by dangerous driving a few months later and an extradition request by the UK, Mrs Sacoolas has refused to return.
US presidents Donald Trump and Joe Biden have both refused to send her back.
If there is no settlement in the civil case, the next step would be a deposition, in which Sacoolas and her husband would give their account of events.
In February, Judge Ellis ruled the substantive claim against Mrs Sacoolas could go ahead in the US, despite the suspect's motion to dismiss it.
Judge Ellis had said: "While it is commendable that defendant Anne Sacoolas admits that she was negligent and that her negligence caused Harry Dunn's death, this does not equate acceptance of responsibility.
"Full acceptance of responsibility entails facing those harmed by her negligence and taking responsibility for her acts where they occurred, in the United Kingdom."
