In the letter, Stephanie Kercher said her younger sister, who was killed in the home she shared with Knox in Perugia on November 1st 2007, was taken away from them "so brutally and unnecessarily".
The family, from south London, will follow the trial from the UK but will maintain close contact with their lawyer, Francesco Maresca.
‘‘Nothing will bring back our beautiful Meredith, but we need to know what happened and she at least deserves the dignity of truth," Stephanie added.
Meredith was on a study year at Perugia's University for Foreigners as part of a degree course in politics and Italian when she was murdered.
Knox and her Italian ex-boyfriend Rafaelle Sollecito were found guilty of the murder in 2009, but acquitted on appeal in 2011.
But in March Italy’s Supreme Court overturned both acquittals and ordered a fresh appeals process.
Knox, 26, has decided not to return for the retrial, saying her stint in prison aged her by 40 years and she suffers from panic attacks and depression. Sollecito, 29, has been living in the Dominican Republic but his family has said he will attend court later on in the trial, which could last months.
A third person, Ivory Coast-born drifter Rudy Guede, who like the other two has always denied the murder, is the only person still in prison for the crime.
Judges on Monday ordered new DNA testing on the alleged murder weapon. The next hearing takes place on Friday.