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 yesterday 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, 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."