The 26-year-old had lived a troubled life up until that point, after his sister Erin was found dead five years earlier in a then-suspected suicide, and a car crash in 2015, which left him with a brain injury and months in rehab learning to walk again.

But his friend Amy Lee said he had been "trying to pick up his life", talking about moving out of his mother's Upper Coomera home and buying a car, when he died in what police alleged on Wednesday was a financially-motivated murder committed by his mother Maree.

A family friend found 18-year-old Erin's body at home in September 2012 while her relatives were away on a cruise.

Joining the dots between the two deaths, police also charged Ms Crabtree, 51, with her murder on Wednesday, alleging the children were harmed over an extended period of time.

The mother-of-three is also accused of the years-long torture of a 25-year-old woman, allegedly beginning interstate when she was a child, who remains in the care of an adult guardian.

She allegedly forced prescription medication on her children in pre-meditated killings, fraudulently claimed more than half a million dollars in insurance benefits, and tried to do the same with more than $100,000 in death benefits from one of the children.

'Goodbye, I'm with her now'

Ms Lee, who became close friends with Jonathan in 2014, said he didn't talk much about his home life but had been close to his sister Erin and missed her terribly.

She said on the night of his car crash, he had sent her a Snapchat which read ???Goodbye, I'm with her (Erin) now".

Just months earlier, he had fronted court after robbing a Gold Coast chemist at knifepoint, reportedly demanding the prescription painkiller Tramadol, before fleeing the store.

His defence lawyer at the time told the court he had become addicted to the drug following another serious car accident in 2009, according to News Corp.

In December 2014, he faced court on a dangerous drug possession charge however the charge was dropped in March the following year.

Speaking to media on Wednesday, Detective Inspector Mark Thompson said police would allege the man had been "counselled" by his mother to rob the pharmacy.

He appealed for information from anyone across the country with details about the family, originally from Sydney, to come forward and help with the investigation.

Financial gain was the end goal: Police

These were not the "compassionate" actions of a mother ending the suffering of her vulnerable children, he said.

"It's extremely traumatic to us as investigators to see what has happened to these children over an extended period of time," he said.

"... I've used the word ???premeditated' and that's exactly what it is.

"It was calculated. It was planned and it was executed with an end goal in mind and we will be alleging that financial gain, significant financial gain, are the motivators for these crimes."

Ms Lee said his mother had been controlling and "had a say on everything he did everyone he saw and spoke to", including while he was in hospital, where she said Ms Crabtree had control over his iPad and iPhone and would go on his Facebook and block some of his friends.

Ms Lee said before the 2015 crash he had been an "outgoing party guy" but after the incident he lost a lot of friends.

Jonathan's former defence lawyer Campbell MacCallum told 7 News Mr Crabtree seemed to be sorting his life out last year.

"He just seemed to be turning a corner which was fortunate at the time and it was a little bit suspicious I suppose that he ended his life at that time," he said.

The former Helensvale Hornets footballer was a "loyal, kind, honest" friend who was always had time for those he loved, Ms Lee said.

In a profile featured on dating website Plenty of Fish, Mr Crabtree described himself as an animal lover and said he was on the disability support pension due to ongoing injuries from his car accident.

He said he had been single for three years and said he wanted children someday.

Police allege his mother took that chance away from him last year after she allegedly forced prescription medication on Jonathan and earlier her 18-year-old daughter Erin in September 2012.

Both adult children were believed to need a higher level of care than most their age, police said.

Police said the children's health and wellbeing was "impacted" prior to their death because of their mother's actions and guidance.

"The family dynamic was harmful to those children," Inspector Thompson said, adding that it was "extremely traumatic to us as investigators to see what's happened to these children over an extended period of time."

Mother accused of insurance payout fraud

Ms Crabtree also stands accused of successfully securing a fraudulent insurance payout of more than half a million dollars and trying to defraud hundreds of thousands more.

Police will allege the mother-of-three attempted to claim a $125,000 Sunsuper superannuation death benefit policy payout between January and August last year, directly linked to one of her children's deaths.

She is also accused of fraudulently trying to claim a $238,800 payout for a Total and Permanent Disability between January and August last year.

In images posted to social media accounts show Ms Crabtree as an animal lover, with her chihuahua Tinkerbell featuring in many of her photos, and a big fan of musician Sia, the reality show Dance Moms and dancer Maddie Ziegler.

If you or someone you know needs support, help can be found at Lifeline on 13 11 14 or Beyond Blue on 1300 224 636.