Warning: Distressing content

The brother of murdered mum Hannah Baxter has revealed his family's desperate attempts to save her from her "monster" husband.

Baxter, 31, died at the Royal Brisbane and Women's Hospital overnight after being doused in petrol by her estranged husband and the children's father, Rowan Baxter, 42.

The children - Aaliyah, 6, Laianah, 4 and Trey, 3 - died in the family car after it burst into flames on a quiet suburban street in Camp Hill in Brisbane's east on Wednesday.

Their father died on the footpath from self-inflicted wounds, police say.

Hannah was taking her children to school from her parents' home when Rowan jumped into the car's passenger seat, according to reports.

The car made it to nearby Raven St where, one witness said, Ms Baxter ran from the car screaming "he's poured petrol on me" as the flames took hold.

Queensland Police say it's too early to say if it was a murder-suicide or an accident.

It is understood Baxter, a former Warriors player in 2005, was in a custody battle with Ms Baxter.

Ms Baxter's heartbroken brother Nat Clarke revealed eight years after the couple had become engaged, married and their romance had turned into a nightmare, his sister had made a hopeful escape from her husband.

"My mum and dad … have given every little thing they own to help my sister try and get away from this monster," he wrote on Facebook.

Hannah's parents Suzanne and Lloyd Clarke told the Daily Mail Rowan was a "control freak" and had kidnapped one of his small daughters on Boxing Day last year and taken her interstate for four days until police returned the child to the Clarke house.

"She had to get the kids and just go without saying anything, because he was such a control freak and would get into her headspace and she would give in," Mr Clarke said, describing an 11-year controlling and abusive marriage which ended last November.

Rowan had sent Hannah a "disgusting" email blaming her and was under a prevention order stopping him from coming within 20m at the time of the ambush murders.

Nat Clarke revealed his sister's last words before Baxter managed to ambush and murder his family.

Clarke said the last thing his sister told his wife was: "I'm so excited, this year will be great."

This was after Ms Baxter had fled Mr Baxter and their marital home, taking their three children to live at her mother, Suzanne Clarke's house in the southeastern Brisbane suburb of Camp Hill.

Close friend Breanna Brown, at whose March 2017 wedding Hannah was bridesmaid, told news.com.au Ms Baxter was devoted to her three young children.

"She would have done anything and everything for those children," Ms Brown said.

"She was literally a super mum."

But it was outside her mother's house on Raven Street, Camp Hill as Ms Baxter prepared to leave on the school run on Tuesday, that Mr Baxter suddenly climbed into her car while the children were in the back.

He soaked his wife and the inside of the vehicle with fuel, lit up the interior and then removed himself from the car.

He then used a knife he had brought to the scene to fatally stab himself.

The children died at the scene with their mum succumbing to terrible burns and dying in Royal Brisbane and Women's Hospital on Wednesday evening.

It has been revealed that after Ms Baxter and her children had fled the home they shared with Rowan, he had become a recluse.

Brisbane resident Debbie Burnett, who was the Baxter family's neighbour, said the killer "was a frequent visitor to my home" and he clearly put on a false front.

Refusing to mention his name, Ms Burnett said: "This monster was my neighbour for three years.

"I am torturing myself trying to recall any signs that I missed.

"I have been rattled to the core because the horror of his actions is something that simply cannot be imagined.

"This murderer was a frequent visitor to my home. You won't hear me say, 'He was a nice guy'.

"Because clearly I didn't know him at all."

Clarke described the father of his murdered nephew and nieces "a heartless monster they called their dad".

"I'm am now trying to help raise awareness about #violenceagainstwomen as monsters like this get away with this to often," Mr Clarke posted on Facebook.

He is trying to raise money for his parents as well as awareness over domestic violence and personal violence against women via a Facebook page, The Clarke family.

"Please share this link or donate if you can so we can give these kids and my sister the funeral they deserve," Mr Clarke wrote with the link.

"RIP you beautiful souls. I'm so grateful she and I got to reconnect and become such close friends before this tragedy.

"I will forever love you all."

WHERE TO GET HELP:

If you are worried about your or someone else's mental health, the best place to get help is your GP or local mental health provider. However, if you or someone else is in danger or endangering others, call police immediately on 111.

OR IF YOU NEED TO TALK TO SOMEONE ELSE:

• 0800 543 354 (0800 LIFELINE) or free text 4357 (HELP) (available 24/7)
• https://www.lifeline.org.nz/services/suicide-crisis-helpline
• YOUTHLINE: 0800 376 633
• NEED TO TALK? Free call or text 1737 (available 24/7)
• KIDSLINE: 0800 543 754 (available 24/7)
• WHATSUP: 0800 942 8787 (1pm to 11pm)
• DEPRESSION HELPLINE: 0800 111 757 or TEXT 4202