Police suspect Erica Coyne and her son Lloyd Mitchell were the victims of a double murder in a violent attack on February 8 in Western Australia.
The 69-year-old mother-of-four was found dead inside her Albany home with serious assault injuries, near to her critically injured son.
Mr Mitchell, 50, later died in hospital from his injuries.
Detectives have been unable to find the pair's alleged killer, prompting the WA government to offer the reward.
Ms Coyne's daughter, Mary Eades, said her mother had a big and kind heart.
"She would give you the shirt off her back," she said.
"She was respected in the community because that's what she showed, respect back to her elders.
"I could accept if my mother passed away naturally, but something like this is like, how do you wrap your head around it?"
Ms Eades said her brother, Mr Mitchell, was a gentle giant who stood up for what he believed in.
Detectives have made extensive inquiries, but the case remains unsolved.
Detective Senior Sergeant Stephen Cleal said it was the toughest investigation he'd been involved in.
"In terms of identifying what has happened, when it's happened, who is involved, there's no doubt there are complexities to this one," he said.
Police Minister Reece Whitby said someone in the community knows something that could help solve Ms Coyne and Mr Mitchell's homicides.
"Now is the time to tell us what you know," he told reporters.
"The families of Erica and Lloyd deserve answers ... and they deserve justice; they are suffering from their grief.
"It might seem like a small detail, but that piece of information could be exactly what investigators need to charge someone to bring them to justice and to achieve a conviction."
Police believe a weapon may have been discarded after the incident.
Detectives have reviewed hours of CCTV and spoken to more than 200 people about the murders.
They have also collected hundreds of pieces of evidence, including several weapons.
The reward is available for anyone with information that leads to an arrest and conviction.
It is the first unsolved homicide in WA with a reward offered since the 2016 murder of grandmother Valeria Fermendjin, according to the Crime Stoppers website.
A $1 million reward is also in place for any information regarding her death.
The 70-year-old dressmaker was murdered in her Perth home in January 2016 during a burglary.
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