Hannah Clarke's brother has spoken of his horror at how his sister and her children suffered as they were murdered, while a women's legal service warns there has been an increase in calls from women saying their partners have threatened to kill them the same way.

Key points: Hannah Clarke and her three children were murdered by Ms Clarke's estranged husband, Rowan Baxter, last week

Hannah Clarke and her three children were murdered by Ms Clarke's estranged husband, Rowan Baxter, last week Mr Baxter doused them with petrol and set them alight

Mr Baxter doused them with petrol and set them alight The Women's Legal Service Queensland says it has had a number of calls from women saying their partners have threatened to do the same thing to them

Ms Clarke and her three children were murdered in Brisbane last week by her estranged husband, Rowan Baxter, who doused them in petrol and set them alight.

In an interview with 7.30, her brother, Nathaniel Clarke, said the thing that "cuts me the deepest" is that Baxter made them suffer.

"It wasn't quick. It was planned and executed," Mr Clarke said.

"He had a plan that night when he called the kids and he was a blubbering mess. He knew what he was doing then. He had it all planned out, he knew what he was doing the following morning.

"He couldn't even do it quick. That's the worst thing. He made them suffer, and her."

Angela Lynch, CEO of the Women's Legal Service Queensland, said the service had been inundated with calls since the incident.

"We've had clients and women saying that their perpetrators and their partner is saying they're going to do what he did to them. They are fearful and something inside them has said, 'I could be next'," Ms Lynch said.

'My heart broke'

Hannah Clarke with her son Trey Baxter. ( Facebook )

Mr Clarke said the day his sister was killed was "the worst day of my life".

"I was out in the paddock working, and the bloke I work with called me in and said, 'Look, you've got to take this, it's personal'," Mr Clarke said.

"I took it and my wife couldn't get the words out. I got: 'You need to come home', and then silence. And my heart broke. I didn't know if it was my kids, I didn't know what was going on.

"And then the next thing she said is: 'He killed them all.'"

Mr Clarke called his father, who told him Ms Clarke was in intensive care.

"I didn't know the magnitude of what had happened," Mr Clarke said.

"It had been explained on the phone what sort of had happened, but I didn't know the whole nation had already stopped."

'He had to control every moment'

Hannah Clarke with Rowan Baxter with their three children, Aayliah, 6, Laianah, 4, and Trey, 3. ( Facebook )

Mr Clarke said looking back, there were little signs over the years that things were wrong in his sister's relationship with Baxter.

"He seemed at the start like a good bloke," Mr Clarke said.

"There were a few things but you didn't see, obviously, what we've seen now. You couldn't have seen that from the start."

He said one of the "small things" was that he and his sister suddenly started having less contact.

"You think something's wrong but you can't put your finger on it," Mr Clarke said.

"You just sort of think, 'Oh, it's just family problems between you and your sister' … but to look back, in a big picture over a timeline, you can now see triggers, I suppose, of, 'OK, this makes a lot more sense now on what he was trying to do'."

Mr Clarke said his brother-in-law's controlling nature was more obvious in the last two or three years.

"He was the type of person that had to win everything, had to be the best at everything, really didn't like when he wasn't," he said.

"He had to control every moment he was in.

"To be one of those people, it does make you selfish, and that was it. It was all about him."

Mr Clarke said he never contemplated Baxter was capable of killing.

"Hannah had her suspicions that he might try to do something to her, but we all thought, 'No, he couldn't'.

"It was just a vicious attack to make her suffer as long as he could. And that was it. It was just — I still can't get over it."

Demanding sex every night 'not normal'

Hannah Clarke and Rowan Baxter on their wedding day. ( Facebook )

One of Ms Clarke's closest friends, Lou Farmer, noticed problems in Ms Clarke's marriage several years ago.

"The way he'd speak to her, and I know other people at the gym saw it, it was just a bit more aggressive," Ms Farmer said.

"The sex every night really got to me because I was just like, 'That's not normal'.

"[She] says she put up with it because otherwise he wouldn't talk to her for days."

Ms Farmer said things began to unravel when Ms Clarke ended the relationship late last year and moved in with her parents.

Ms Clarke took a domestic violence order out on Baxter after he kidnapped one of their children for four days in December.

"He was one step ahead of her all the time. He knew conversations that she hadn't talked to him about — maybe us or another person — he knew of it and prompted her, so he had some recording," Ms Farmer said.

Ms Farmer believes Baxter must have had "some kind of device" to spy on Ms Clarke.

'Last thing we said was I love you'

Nathaniel Clarke is supported by his father Lloyd Clarke and Councillor Fiona Cunningham during his speech at the vigil for her and her three children on Sunday. ( AAP: Sarah Marshall )

Asked if he felt the system had failed his sister, Mr Clarke said he did not.

"I think the system that they've got is quite good, I just don't think it accommodates for people like him," he said.

"Through this whole experience she had amazing support from the police. They did absolutely everything they could.

"She did everything she could to fight him in the legal system but she knew, just from the feeling herself, that she felt supported but not safe."

The last time Mr Clarke saw his sister was a recent day out at Sea World, where the siblings had both taken their kids. As they left the theme park they hugged.

"The last thing we said to each other was, 'I love you'," he said.

"That was the one thing my parents always brought us up to do.

"It doesn't matter if you're on the phone at work to your dad, we'll always say it."