The stepsister of Harry Dunn has called on the Northamptonshire Police chief to resign after a "disgraceful" tweet he made about her family.
Nick Adderly, Northamptonshire Police Chief Constable, commented on the civil action the family is taking against the woman suspected of killing Mr Dunn, Anne Sacoolas, and Donald Trump, saying: "How sad but how predictable.”
Mr Adderley has since deleted the tweet and apologised.
In an interview, Larna Harber criticised Mr Adderly's remarks and said the handling of the situation by police had been poor.
Speaking about the tweet, she said: "We were absolutely outraged by it and disappointed. Its our police force that is meant to be supporting us and when he tweeted 'so sad but predictable', you lose a little bit of faith in them.
"As his role, he shouldn't even be tweeting such things. They've gone against us when we're the ones who are suffering here. They are supposed to be on our side, supporting us and guiding us through the next steps.
She added: "After his comment, its just disgraceful that someone in his role can comment on such a thing when he should be supporting us and we have asked that he resigns from his role."
Ms Harber also criticised Northamptonshire Police's decision to interview Ms Sacoolas while she remained in the US.
She also said her family were only informed of the interview once it had taken place.
She said: "We don't feel it was right that Northamptonshire Police come over to the US to be interviewed.
"She said that her reason for asking them to come over was so they could see the devastation it had caused her and her family."
She added: "If she thinks her family are suffering, what the hell does she think we're going through?"
Anne Sacoolas is believed to have been driving on the wrong side of the road when she hit 19-year-old Mr Dunn's motorbike outside RAF Croughton in August.
Radd Seiger, a spokesman for the family, said the tweet was "disgraceful".
"[We were] shocked, disappointed, frustrated, angry but not surprised," he said. "It's adding insult to injury...but that was the straw that broke the camel's back."
In a statement issued by Mr Adderley, he said: "The tweet was in reference to the profound sadness and loss of Harry Dunn, that the spokesperson for the family has been absolutely clear on the next steps in fighting for justice for Harry and that the only recourse for justice that they feel that they have available to them at the moment is through the US civil court system.
“I deleted it with a view to re-writing it with more context but was called away to deal with something else.
“I meant no offence by my tweet and apologise if it has been in any way misconstrued or misinterpreted."
However the family have called on Mr Adderley to resign following the tweet and said in a statement it shows a "staggering degree of unprofessionalism."
Mr Seiger added: "He has behaved disgracefully, it was unprofessional of him to do so, he had urged everybody at a press conference, including me, to show restraint in public commentary and he also said it was his intention to remain totally impartial.
"Well it's difficult to think of a more partial, biased tweet, which obviously he just blurted out, he then subsequently deleted it quickly, but the damage was done.
"He doesn't want this case around, he doesn't respect the family...and they are absolutely insistent that he has to go."
The family are expected to refer Northamptonshire Police to the Independent Office for Police Conduct over their handling of the investigation.
Northamptonshire Fire, Police and Crime Commissioner (PCC) Stephen Mold said he has spoken to Mr Adderley about keeping force communication “professional and sensitive at all times”.
Meanwhile, detectives investigating the death of Harry Dunn have interviewed the suspect in the case in the US.
PA reported Northamptonshire Police have passed the details of the interview to the Crown Prosecution Service (CPS) for consideration.
Mrs Sacoolas, the wife of a US diplomat, was granted diplomatic immunity after the case, which has since been disputed by the lawyers acting on behalf of the teenager's family.