The sister of murdered backpacker Hannah Witheridge has warned that Thailand is a “dangerous trap”, suggesting her sister’s killer walks free and slamming the Thai police and government for mistreating her bereaved family.

In a long Facebook post, Laura Witheridge said “the vast majority of the Thai police are corrupt” and the investigation that led to the death sentence last month for two Burmese men was “bungled”.

She also hinted she believed Thai, not Burmese, nationals killed her sister on Koh Tao island and she received death threats throughout the trial, including one saying the killers had completed only “half the job”.

“What if I told you that I am now frightened of my own shadow?” she wrote.

Witheridge said she was compelled to write the public post after an “ignorant (Facebook) comment someone had made about how Thailand is the most beautiful place in the world”.

“Aesthetically, on a postcard or photograph, maybe,” she said. “Aesthetic beauty can lure you into a very dangerous trap.”

When her family visited in 2014, Witheridge said, Thai police offered to provide them with an official update at their headquarters but “200 journalists were allowed into the room and we were ambushed by this mob of hungry journalists shoving cameras in our faces”.

She added: “The Thai police chief had no intentions of giving us an update … after all, the bungled investigation meant he had nothing to tell us. The invitation was merely an opportunity for the press to take photographs of our family.”

The family was later “absolutely appalled” when “the Thais offered us ‘compensation’”.

Hannah Witheridge, 23, from Norfolk, was found on Sairee beach on Koh Tao alongside 24-year-old David Miller, from Jersey, in September 2014. Witheridge had been raped and bludgeoned to death and Miller had been attacked and left to drown in the surf, Thai police say.

Laura Witheridge said that, since then, there have been many more suspicious deaths on Koh Tao, including Briton Luke Miller – unrelated to David Miller – whose body was found in a pool last week. Thai police said he drowned while heavily drunk.

“You probably haven’t heard of them all, as not all were British nationals. The deaths, where possible, are covered up as suicides and accidents. This would have happened with Hannah, if it had not been for the hideous brutality of her passing.”

Witheridge said that in the recent case of Luke Miller, she suspected Thai authorities would say it was an accidental death caused by drugs. “Hiding the truth and offering a story that suits, is something that they do often,” she said.

The Witheridge family neither supported nor condemned the sentencing of migrant bar workers Zaw Lin and Wai Phyo on 24 December. Laura’s Witheridge’s post provides a stark view of the concerns the family hold over the entire investigation, which has also been widely condemned by rights groups.

The Burmese pair initially confessed to the murder but later rescinded their statements, saying they had been tortured by police to admit to the brutal beach attack.

The Thai police, who deny any wrongdoing, were also accused by the defence team of mishandling crucial DNA evidence. Royal Thai Police spokesperson Dejnarong Suthicharnbancha told the Guardian the authorities were not aware that Laura Witheridge was receiving threats.

“We need to question her regarding this issue and launch [an] investigation. And we can assist her if she wants to do anything in this matter,” he said. He added that Witheridge’s allegations of corruption were presented “without evidence and without fact. If there’s any corruption, it has to be seen on a case by case basis.”

David Miller’s family were at the sentencing and told reporters afterward they believed the police investigation and forensic work “was not the so-called shambles it was made out to be” and said justice was delivered.

Laura Witheridge said she had been attacked online by Thai people, who sent her graphic crime scene photographs.

In her Facebook post, she hinted that though the murderers were Thai nationals: “What if I told you that I have had many death threats from Thai people since they murdered my sister? That they defaced photographs of me saying that the killers had only done ‘half the job’ … what if I told you that people commented on these photographs saying things like ‘there is still time’, and ‘tick tock tick tock’.

“What if I told you that I have been chased in my car?” she added, without giving details. “I miss my sister desperately. My heart is heavy and my mind is tired.”

She said many people she and her sister knew continue to travel to Thailand on holiday. “They think it wont happen to them … Well, guess what? Neither did we. No one is immune. Many Thai’s hate westeners [sic] and they have little to no regard for human life. I don’t say this lightly, or without reason.”

Witheridge said that those who still think Thailand is a safe travel destination should watch a recent video produced by the cyber activist collective Anonymous that accuses the Thai police of a cover-up in the Koh Tao and other murder cases.

Anonymous said the Thai police “would rather blame foreigners or migrants for such crimes so as to protect their tourism industry than accuse their own Thai locals, that may deter tourists from choosing Thailand as their holiday destination”.