Mum-of-six Sarah Barrass, 35, and her family member Brandon Machin, 37, are both facing life in jail after pleading guilty to killing Sarah's boys Blake, 14, and Tristan, 13.
The murders took place in May this year in Sheffield, UK. The brothers were strangled to death after a failed attempt to poison them the day prior.
Now, Sarah's brother Martyn Barrass, 34, has spoken to The Mirror, saying his sister and his family member Brandon have always been evil and he warned social services that the kids were in danger 10 years ago.
'Those two have been evil since birth. They are both as bad as each other, they are both murdering, evil psychopaths,' Martyn told The Mirror.
'I witnessed Brandon’s violence towards Blake years ago and I knew something wasn’t right.. I called social services and asked them to look into it. If they had, the boys might still be here today.'
Martyn, a father of twins, says he called social services three times but felt like he was ignored.
Court documents tell how Barrass and Machin tried to kill all six of her children over a horrifying 48-hour period. As well as trying to poison Tristan and Blake before strangling them, an attempt to drown another child failed.
Tristan and Blake were found by paramedics in a critical condition at their home at 7:30am on May 24 and taken into hospital, but they later died just 12 minutes apart.
Barrass' four other children, who can't be named for legal reasons and are all under the age of 13, were taken into hospital but released the same day.
'Blake and Tristan were polite, kind-hearted lads. It’s a tragic waste of life,' Martyn told The Mirror.
Martyn claimed that as they were growing up, Barrass and Machin would hunt pet mice for fun and watch violent horror films together. They also taunted and bullied Martyn for fun.
Ten months before she murdered her own children, Barrass chillingly posted a Stephen King quote meme to Facebook that read: 'Murder is like potato chips; you can't stop with just one.'
On September 27, Sarah Barrass, 35, and Brandon Machin, 37, appeared at Sheffield Crown Court and pleaded guilty to two counts of murder and one count of conspiracy to murder six children and five counts of attempted murder.
Judge Jeremy Richardson QC said: 'The crimes you have committed quite frankly speak for themselves. The murder of two children.
'The attempted murder of four children and the over-arching conspiracy to murder those children.
'I repeat, those crimes speak for themselves.
'I have little doubt that each of you will in due course be sentenced to several terms of life imprisonment.'
They are both due to be sentenced on November 12.