Nearly three months after 19-year-old Harry Dunn was killed in a head-on collision in England, lawyers for his family are suing the Trump administration, as well as Anne Sacoolas, the wife of an American official who admitted to causing the car crash.

Harry's stepsister, Larna Harber, told CBS News it's a step they never wanted to take.

"In simple words: Get her back on the plane to the U.K. Donald Trump has the authorities to do that and I can't understand why they're not," she said. "She's a mom of three children — what example is this setting to her three children? … It just seems wrong. I don't understand why when something seems so simple to us."

Sacoolas' immunity is a complex issue — one that reached the highest levels of government in the U.K. and led Harry's parents all the way to the White House, where they say they were "shocked" when President Trump asked them to meet with Sacoolas who was in an adjoining room.

They declined that meeting, saying they would only meet with her in the U.K. and only in the presence of grief counselors. That still hasn't happened.

The family said it will accuse the administration of "lawless misconduct" and will also be seeking damages. CBS News reached out to Sacoolas' lawyers and is waiting for a reply.