The family of Harry Dunn, the 19-year-old motorcyclist killed in a crash last August, is to urge the foreign secretary to try Anne Sacoolas, the American woman charged with causing his death by reckless driving, in her absence.

The US confirmed last week it had taken the rare step of rejecting a British request to extradite Sacoolas, on the grounds it would undermine the principle of diplomatic immunity, which it said Sacoolas was entitled to as the wife of an American intelligence officer who had been working in the UK.

The US government flew Sacoolas and her family back to the US days after Dunn died following a collision with a car that was allegedly being driven by Sacoolas on the wrong side of the road near RAF Croughton in Northamptonshire.

The Dunn family is due to meet Dominic Raab at the Foreign Office (FCO) on Monday and also wants the FCO to drop its claim that Sacoolas did have immunity at the time. The family is launching a judicial review to challenge how the UK government reached this view on her immunity, but the FCO is resisting the review, partly by threatening the family with substantial costs if it loses the case.

The meeting comes as both Raab and the prime minister, Boris Johnson, prepare to meet the US secretary of state, Mike Pompeo, in London on Wednesday.

Charlotte Charles, Dunn’s mother, appealed to Johnson to stand up for the UK in the fight for justice.

The UK has described the US’s refusal of the extradition request as a denial of justice, but with many wider diplomatic disagreements with the US already clouding the “special relationship”, it is unclear how much pressure the FCO will put on Donald Trump to reverse the decision.

Charles issued an appeal to the prime minister on the BBC’s Victoria Derbyshire Show, saying: “Boris Johnson you are the one who has got the power in your hands. You are the one that can bring an end to this. Do the humane thing and make your country proud of you. Show us you are willing to stand up for your citizens’ rights and bring her back.” He could show that “we are as equal to them and as powerful as them”, she added.

The family spokesman, Radd Seiger, said Dunn’s constituency MP and also the business secretary, Andrea Leadsom, supported the idea of trying Sacoolas in her absence. He said such a move was not legally unprecedented. “The jury would be made aware that she chose not to show up,” he said.

He also accused the government of “fighting these poor parents tooth and nail in this judicial review over whether she had diplomatic immunity at the time of the crash. Both the US and US governments say she did.” He insisted international lawyers challenged that view as a misreading of the Vienna convention on diplomatic relations.

He said he did not want any high-fiving between Pompeo and Raab this week, saying instead the UK had to tell the US “it is not OK for you guys to come to our country, kill our citizens and then get on the next plane home”.

Charles also questioned how Sacoolas could “get behind the wheel of a car in the US and not see our boy on the windscreen”. She said she could not understand how Sacoolas could think she could set such an example to her children by evading justice for her mistakes.

Speaking of the continued anguish in her family, she said Harry’s twin brother, Niall, “was no better”. She said: “He has been through a particularly rough time. He has not accepted it at all. He is in denial. He knows Harry has gone, but he does not see the point of going forward with anything because Harry is dead. Struggling is an understatement.”