Amanda Knox is refusing to return to Italy after judges reinstated her murder conviction for the death of British student Meredith Kercher.
Knox was sentenced to 28 years and six months and her former boyfriend Raffaele Sollecito was jailed for 25 years.
He was held by Italian police at around 1am today and had his passport seized after reportedly taking a short trip to Austria.
Miss Kercher's family have called on Knox to be extradited from the US, but she insisted: "I will never go willingly back."
The victim's brother, Lyle, told a press conference in Florence that it would be "strange" if the 26-year-old was not extradited.
''If somebody is found guilty and convicted of a murder, and if an extradition law exists between those two countries, then I don't see why they wouldn't", he said.
''I imagine it would set a difficult precedent if a country such as the US didn't choose to go along with laws that they themselves uphold when extraditing convicted criminals from other countries.
''It probably leaves them in a strange position not to.''
But Knox told ABC's Good Morning America: "I will never go willingly back ... I'm going to fight this to the very end. It's not right and it's not fair."
She told the US show that she has sent a letter to her lawyer which is addressed to Miss Kercher's family.
"It's in the mail. Mainly I just want them to know that I really understand that this is incredibly difficult, that they've also been on this never-ending thing and, when the case has been messed up so much, like, a verdict is no longer consolation for them," she said.
But the victim's sister, Stephanie, said her family did not want to read the letter.
"I would say no because I don't have a need to read it at the moment," she said. "At this stage, as you know, it's not finalised, the decision, so until the end of the trial when we know exactly where we stand with who has definitely been confirmed as guilty or not guilty, it's very difficult to try and understand my own feelings as to whether I would want to read that letter or not.
"So for now I would say no."
She also revealed that her family do not want to meet Knox, telling reporters at the Florence press conference: ''It's not something that we would want to do at the moment and I can't say that we ever will."
Sollecito was found with his current girlfriend at a hotel in Venzone, near Italy's border with Slovenia and Austria, in the early hours of this morning.
He was taken to a police station in Udine where he was told he was not allowed to leave Italy and his passport was confiscated.
According to Italian news agency ANSA he explained to officers that he had been driving in Austria before coming back to Italy.
The 29-year-old told his lawyers: "I never thought of fleeing, not in the past nor now."
Miss Kercher, a 21-year-old Leeds University exchange student from Coulsdon, south London, was found with her throat slashed in the bedroom of the house she shared with Knox in Perugia, central Italy, in November 2007.
Knox and Sollecito were originally found guilty of murder in 2009.
They were cleared nearly two years later - but the appeal court ordered a fresh trial last March.
Prosecutors claimed that Miss Kercher was the victim of a drug-fuelled sex game gone wrong, but the defendants have consistently protested their innocence and claim they were not in the apartment on the night she died.
Rudy Guede, a drug dealer, is serving a 16-year sentence over the death - though the courts have said he did not act alone.
In an interview before the verdict was handed down, Knox told the BBC: ''They'll have to catch me and pull me back, kicking and screaming, into a prison I don't deserve to be in.''
Speaking outside the court, her lawyer, Luciano Ghirga, said he would launch an appeal.
"For those that, like me, are convinced that Amanda is innocent, it is a very difficult time,'' he said. "We have to respect the verdict but we will challenge them."