A Birmingham academic has called the case of an incestuous couple who murdered their teenage sons the 'most horrific' case she has ever studied.
Half-siblings Sarah Barrass and Brandon Machin, both of Sheffield, were sentenced to life in prison with minimum terms of 35 years on Tuesday.
Prof Elizabeth Yardley, a criminologist at Birmingham City University, said: "In a lot of cases of child homicide the parents don’t last very long in prison.
"They are a very high risk of suicide and they are at a very high risk of attack.
"I doubt very much they will live to see the end of their sentences."
Teenagers Tristan, 13, and Blake Barrass, 14, died after being strangled by the pair at a property in the Shiregreen area of Sheffield, and having bin bags placed over their heads.
Sheffield Crown Court heard they feared the children would be taken into care if their incestuous relationship was discovered and decided the children would be "better off dead".
But Prof Yardley said said believed the murders didn't happen just "out of the blue" and the incestuous couple put up a facade to hide a "highly dysfunctional" family.
She said: "This is, I think, the most horrific one I have come across in my time as a criminologist.
"It is horrendous. It’s like something from a horror film. You could not make this stuff up."
Prosecutors told how Barrass had asked for help with the children from the local authority with the children, texting a friend to say: "I've thought of every possible solution to this mess. Mass murder, putting them all in care, checking in to the local nut house.
"I love my kids too much to kill them, I can't put them into care for the same reason."
Kama Melly QC, prosecuting, said visitors to the children's home heard Barrass tell her children: "I gave you life, I can take it away."
Sheffield Crown Court heard how, prior to the killings, Barrass and Machin had given four of her children - including Tristan and Blake - tablets against their will.
The pair also tried to kill one of Barrass' children by attempting to drown them in a bath.
After reviewing the case, Prof Yardley said she did not believe claims that Barrass and Machin plotted the murders because they feared the children would end up in care.
She explained: "I think that is nonsense.
"That has been the case for years, a risk of this (secret sexual relationship) coming out ever since the first children were born, so why now? Why choose to kill them at this point?
"I think there's more to it.
"I’m totally speculating here, but if you look at the order of events they have killed the eldest children first, and the eldest children represent the biggest threat and the biggest risk if they (Barrass and Machin) were going to disclose some information that would compromise the parents’ control of the children.
"That’s what this all boils down to is control.
"I’m really sceptical of the narrative that has emerged around this case."
As the killers were jailed, a judge lifted a reporting restriction which banned the media from identifying Machin as Barrass' half-brother and the father of her six children.
As for what the incestual relationship said about the parents, Prof Yardley said: "It’s about the normalisation of the abnormal. For them, the weird was the every day.
"They’re acting beyond the boundaries of the mainstream norms.
"It’s a damaging foundation on which to build a family."
The four surviving children, who cannot be named for legal reasons, are all aged under 13 and are being supported by the local council.
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John Macilwraith, executive director of People Services at Sheffield City Council, said: "Our focus continues to be the well-being of the children from this family now in our care.
"They are safe and receiving good support through this traumatic time.
"The Sheffield Children Safeguarding Partnership has initiated a Serious Case Review (SCR) to investigate what happened."
