The Courts of Justice and police have dismissed condemnation from Laura Witheridge, the sister of Koh Tao rape and murder victim Hannah Witheridge, concerning the investigation and trial.
They said every step of the probe was carried out transparently under the Thai justice system.
Ms Witheridge claimed that judges and court officials inappropriately said: "Why are you here? Why do you care? She [Hannah] is dead already."
"Just go home and make another one [daughter]. She will be back in 30 days as something else, she may have better luck next time," Ms Witheridge also quoted Thai officials as saying to her parents.
Suebpong Sripongkul, spokesman of the Courts of Justice, said Tuesday the trial of the case was transparent and public to all parties concerned, including the victim's family members.
"If the allegation [about the court's inappropriate manner during the trial] was true, such information should have already come out," he said, adding that if the sister has information as to which judges or court officers acted inappropriately, she could file a petition via the British embassy.
Ms Witheridge was writing on her Facebook page on Sunday after learning about the death of another British national, Luke Miller, 27. Miller was a tourist who was found dead in a hotel swimming pool last week.
She also wrote that she received several death threats from Thais and was sent defaced photographs of herself.
Ms Witheridge claimed that her family was offered -- and refused compensation to keep quiet about the murder of her sister.
She also said her family was enticed by Thai police to their headquarters with the promise of updates on her sister but were instead paraded in front of the press.
Royal Thai Police deputy spokesman Songpon Watthanachai said police believe in fair court proceedings, adding the ruling was based on actual facts and evidence collected from the crime scene.
"No officials want to destroy their own country's reputation," he said.