Neighbours of the incestuous half brother and sister who murdered two of their children suspected they were a couple when the youngest kids called him 'dad', MailOnline can reveal.

Sarah Barrass and Brandon Machin had a carefully constructed back story to mask the shocking truth that they were lovers and that he had fathered her six children.

But neighbours in the road where Machin claimed to be bringing up her children as a single mother were suspicious of the amount of time her 'brother' was spending at the property.

Their suspicions grew when Machin would make a show of leaving the house at night, only to sneak back in under the cover of darkness.

The evil pair were jailed for 35 years yesterday for strangling their son Tristan, 13, in front of 14-year-old Blake before killing him as social services edged toward rescuing them from their chaotic home life in Sheffield.

After the 12-minute murder spree they attempted to drown a third child in the bath and planned to kill the other three using drugs overdoses, two of whom were under three years of age, but Barrass then called the police.

The horrific case raises major questions about who knew the truth about their relationship and how they kept an illegal sexual relationship secret despite being on the radar of social services since the 1990s.

Neighbours said today that people in the street knew of the relationship taking place under the noses of Sheffield City Council, who has launched a serious case review.

One told MailOnline: 'When Sarah first moved into the house she only had the four eldest children and Brandon appeared on the scene straight away.

'She was at pains to point out to everyone that she was a single mother and had four kids to look after and introduced him to everyone as her brother.

'That was accepted at first even though it seemed strange he spent so much time at the house with her and the kids when there didn't seem to be any need for him to be there.

'She had two more children and yet there was no sign of a father and no explanation for how the little ones came to be conceived. The only man who was ever seen at the house was her brother.

'My suspicions really started when I would watch him leave the house and shout goodbye, he'd see them in the morning.

'I'm a light sleeper and would notice that a while after leaving he'd come back when it was dark having walked around the block. He was sneaking back in when he thought no one would see.

'It seemed really strange and I didn't know what to think. Your mind looks for answers but they're too strange to dwell on.

'Then after the younger children were born and could talk they'd slip up and call him 'Dad.'