Police have offered a million dollar reward for any information that can help them solve a 30-year-old murder case.
Doris McCartney, 71, and her brother Ronald Swann, 69, were murdered in their Keith Street home in the Melbourne suburb of Moorabbin on 22 October 1989.
Both had been assaulted, but investigators have been unable to discover why they were killed.
"Rarely do I feel so strongly that the release of a reward can actually solve a murder," Homicide Squad Detective Inspector Tim Day told reporters on Tuesday.
Cold case detectives reviewed the file two years ago and are now following "active lines of inquiry".
The pair was last seen alive at about 6pm Saturday 22 October 1989, with a friend of Doris' visiting their house the following morning at about 9am and discovering their bodies.
Inspector Day said on the night of the murders there was no sign of forced entry into the home, and nothing had been stolen.
"This was not a random attack at all ... we now believe the offender was known to Doris, and we now believe the murder stemmed from that association.
"There's a person of interest that we believe had an association with Doris and not Ron, the reason for that belief is something that I can't speak to you about," Inspector Day said.
He asked anyone with information to come forward, even if it seemed insignificant.
"To us it may well just be the last piece of the jigsaw... people who commit most crimes don't generally keep it secret," he said.
Doris McCartney had been deaf since the age of 15, and had difficulty communicating with people.
After her husband died in 1979 her brother Ronald Swann moved in to help take care of her.
One of Mrs McCartney's three children, Ian McCartney, said the long wait for answers had been a strain on the family.
"There's not a day in 31 years that I haven't thought about it ... it's not easy," he said.
He told reporters the family used to celebrate Christmas with a sit-down lunch for 45 people, but after the murders found they could no longer celebrate the occasion.
"It made Christmas an unhappy occasion, which I didn't think was fair," he said.
Mrs McCartney's daughter Patricia Newman asked whoever is responsible to come forward.
"Please, I beg you ... use your conscience before you go," she said.
Police have yet to arrest anyone regarding the case.