Her body was found in a septic tank at a hotel in central Kenya nearly three months after she had spent an evening partying with soldiers.
The 21-year-old left behind a five-month old daughter, who is now being cared for by her sister Rose Wanyua in the town of Nanyuki, 200km (125 miles) north of the capital, Nairobi.
Ms Wanyua sobbed as she revealed how the Sunday Times report had raked up painful memories.
She and her husband John Muchiri said the family had given up any hope of finding justice for Ms Wanjiru, who they knew as Ciru.
“But whoever killed her went free and is living his life. I am raising her child alone, no-one has asked about their well-being, not even the government.”
‘We can’t afford lawyers’
Ms Wanjiru, who dropped out of high school and later became a sex worker to look after her baby, was last seen by witnesses on the night of 31 March 2012.
She was walking out of a Nanyuki bar accompanied by two British soldiers.
Her body was later discovered behind a room where the soldiers had stayed, with missing body parts and a stabbing injury.
Kenyan judge Njeri Thuku concluded after an inquest in 2019 that Ms Wanjiru had been murdered by one or two British soldiers.
She ordered two further criminal inquiries, but the military took no action, the Sunday Times reports.
A soldier told the paper that the killer had confessed to him and he had reported it, but the army had failed to initially investigate.
Kenyan detectives are also said to have asked British military police to question some of the soldiers, but the UK’s defence ministry denied receiving such a request.
On Tuesday, UK Defence Secretary Ben Wallace said the Ministry of Defence (MOD) had fully co-operated with the Kenyan investigation – and would continue to do so.
Mr Muchiri said the family felt let down, but could not afford legal representation to take the matter further.
“You know, we are poor. I’m a casual labourer. We’d have loved to retain a lawyer to follow up. We can’t,” he said.
For decades, locals have complained about the British army, which has a training base in Nanyuki.
A lawyer who represents them told the BBC many of the issues go unaddressed.
“Even with our local police, you will just report a case, and then nothing goes beyond the investigation stage,” Kelvin Kubai said. “Most of the locals do not have the resources and financial capacity, to enable them pursue justice on such grounds.”