Knox, who lived with Kercher in Perugia, Italy, was initially convicted in Kercher's murder in 2009, then was acquitted in 2011, at which point she returned to the United States, Italy's ANSA news agency reported.
On Thursday, Knox and her ex-boyfriend, Italian Raffaele Sollecito, were re-convicted in the case and sentenced to 28 and 25 years in prison, respectively. The court did not order them held because they have appealed their cases.
It is unclear whether the United States will agree to extradite Knox, 26, from Seattle, if her conviction is upheld again.
Kercher's brother, Lyle, told Britain's Daily Mirror he believes Knox "should be locked up while the process goes on."
"I don't understand why a convicted criminal is not in custody. America needs to practice what it preaches," he said. "We see cases all the time where they're trying to extradite people -- Julian Assange, Edward Snowden -- [back to the United States]."
"You can't be going around saying one thing and doing another. I would be surprised if America doesn't accept it if Italy calls for extradition. It would make a mockery of the word reciprocation."
Meanwhile, Sollecito, who has been ordered to remain in Italy and has had his passport seized, said Monday he believes he should have been questioned in Knox's case, ANSA reported.
"If indeed my statements could have changed the course of the trial, why didn't anyone feel the need to interrogate me?" Sollecito said. "I was available but in all these years no prosecutor or judge has ever [requested testimony]."