Legal advisers to Bamber believe the graphic police pictures of Sheila Caffell, which remained undisclosed during his trial and two subsequent appeals, will provide the grounds for a third appeal.
Bamber, 44, has always denied shooting his adoptive parents, Neville and June Bamber, along with Caffell and her twin six-year-old sons at the family's farmhouse in Tolleshunt D'Arcy, Essex, in 1985.
The new evidence, which has been submitted to the Criminal Cases Review Commission (CCRC), is said to prove that Bamber was in police custody when Caffell, 27, was killed.
Scientists will be called to argue that apparently fresh blood on Caffell's body means she can only have died a maximum of two hours before the pictures were taken at around 9am on the day of the murders.
A log of police activity also records that blood was seen flowing from the mouth of Caffell, a former model nicknamed Bambi - at about 7.30am.
Police records show that Bamber, then 24, had been with police since about 3am after he had called officers to tell them about a disturbance at the farmhouse.
Giovanni Di Stefano, the high-profile Italian legal adviser acting for Bamber, said he believed that the pictures were enough evidence to quash his client's five convictions for murder.
Mr Di Stefano, whose clients include the killer Kenneth Noye and the timeshare conman John Palmer, said: "These photographs prove, at the very least, that Mr Bamber could not have killed his sister.
"We are instructing experts on this but anyone knows that if blood is leaking at 7.30am as an officer's statement said, and the pictures show that, she could not have been dead at the time he was taken into police custody."
The new evidence is understood to come from a large cache of about 500,000 documents from the police investigation which were not disclosed to defence or prosecution lawyers during Bamber's original trial or his two failed appeals in 1987 and 2002.
During the trial, jurors were told that Bamber had shot his victims at White House Farm before placing the rifle and a Bible on the chest of Caffell to make it look as if she had carried out the killings and then committed suicide.
Caffell was initially the chief suspect after detectives were told by Bamber that she suffered from schizophrenia and had not been taking her medication.
The judge described Bamber, whose apparent motive was a £436,000 inheritance, as "warped and evil beyond belief".
A statement made by the first police officer on the scene, PC Peter Woodcock, described how he had entered the farmhouse at 7.34am and found Caffell's body. The officer said: "She had what appeared to be two bullet holes under her chin and blood leaking from both sides of her mouth down her cheeks."
Mr Di Stefano said that together with the photographs the evidence showed that Caffell could not have been dead when his client met police at 3am.
Bamber has always claimed that on the evening before the shootings there had been a row between his parents and Caffell over whether her son should be taken into care. He said he then received a call from his father in the early hours saying his sister had gone mad before the phone went dead.
Mr Di Stefano said other discrepancies had also come to light from the undisclosed evidence, including indications that Caffell's body had been moved after her death.
The CCRC confirmed that it had reopened its file on Bamber and was considering the new evidence.Reuse content