The twin brother of Harry Dunn, the British teenager who died in a road traffic collision with the wife of a US intelligence officer in August, has appealed for her to come back to the UK.

Niall Dunn did so as lawyers for the family travelled to the US to see if a civil action against Anne Sacoolas could be mounted. The week-long lawyers’ visit coincides with a separate trip by Northamptonshire police who are planning to interview Sacoolas over the accident. She has admitted involvement, but is claiming diplomatic immunity.

According to the Foreign Office, the US state department has said her diplomatic immunity is no longer pertinent, but no elaboration has been given of what this phrase means in practical terms.

In an interview for the Mail on Sunday, Niall Dunn said there had been thoughts he had not been able to process properly about the death of his fraternal twin. Housebound with grief, he said he had been proud of the campaign his parents had mounted to find out the truth about why Sacoolas had been allowed to return to the US and avoid further UK police inquiries.

Sacoolas allegedly killed Dunn when driving on the wrong side of the road as she left RAF Croughton in Northamptonshire, a US listening base on 27 August. She had only been in the country for a few weeks and admitted responsibility to the police in initial questioning.

Niall Dunn appealed to Sacoolas, saying: “We want for this to be over, to live our lives the best way we can. We can’t do that unless you cooperate and help us. This all could have been avoided if you had just stayed here and done the right thing. Although that time’s passed you can still put an end to all of this.”

He added: “I’m not happy with how everything has happened. I’ve wanted to start grieving but now it’s become so huge that I have to be involved.

“When we were told Anne Sacoolas had diplomatic immunity it was a punch in the face. ‘Tough, she’s not coming back, end of story. Go away and cry at home.’ That’s what it really did feel like. It was really difficult hearing that and knowing we had nowhere left to go, nothing left to do. They left us to just cry and grieve in silence. I’m angry at the higher-ups of the world who just don’t seem to care at all about what’s happening.

“It was swept under the carpet and that’s what really got to me. It made me angry. Harry lost his life. We’ve lost everything. I want to know who decided this was OK to do this to us.”

Q&A How does diplomatic immunity work? Show Diplomatic immunity is the protection given under international and UK law to foreign diplomats and their families. It was formalised through the 1961 Vienna convention on diplomatic relations (VCDR). Reciprocal global agreements also protect UK diplomats working abroad. Without a foreign state agreeing to lift immunity, diplomats and their dependent families can only be detained “as a last resort” if, for example, they are deemed to be a danger to others or themselves. The only other sanction available to the Foreign Office would be to order their expulsion. When a foreign state agrees to remove that legal protection, it is known as a “waiver of immunity”. Individual diplomats cannot do so; the embassy of the foreign state has to make a formal request. As well as covering diplomats and their families anywhere in the UK, the same convention and legislation prevents UK officials from entering diplomatic premises. That inviolability enabled Julian Assange to stay in the Ecuadorian embassy in London for seven years. Owen Bowcott, Legal affairs correspondent

In the interview Niall Dunn revealed that the police had wanted to charge Sacoolas with death by dangerous driving, which could have meant a custodial sentence. But six days after Harry’s death, unaware that there were questions already being raised behind the scenes over Sacoolas’s diplomatic status, the family requested the charge was reduced to death by careless driving, which usually involves a suspended sentence and would avoid separating her from her children.

A week later Sacoolas took a flight out of the UK claiming diplomatic immunity. The family were not informed until 10 days later, and there are now serious questions over whether the Foreign Office’s advice to the police – namely, that Sacoolas was entitled to such immunity – was correct.

The claim rests on a written agreement – currently not in the public domain – between the US and the UK over the status of staff and their dependents at the base.

• This article was amended on 11 November 2019 to omit an unconfirmed reference to Sacoolas having taken a commercial flight out of the UK.