A 21-year-old man has been sentenced to 20 years to life in prison after admitting suffocating his three-year-old stepsister.

Brian Scales, from Trotwood, Dayton, pleaded guilty to two counts of murder, aggravated murder and child endangering on Wednesday.

Scales admitted strangling Tristan Carlton on February 14 this year, the Dayton Daily News reported.

Brian Scales (pictured left), from Trotwood, Dayton, has been sentenced to 20 years to life in prison for suffocating his three-year-old stepsister Tristan Carlton (right)

He had been babysitting the little girl and her eight-year-old sibling at the time.

The toddler’s body was found ‘in her bed, cold and stiff’ by their mother Inas Scales when she returned home from shopping that afternoon.

Medics attempted to revive the youngster, but were unsuccessful.

Her stepbrother was charged after the child’s mother called police and an investigation into the death revealed she had been suffocated.

Scales pleaded guilty to two counts of murder, aggravated murder and child endangering at Montgomery County Court (pictured) on Wednesday

But at the sentencing, Inas Scales told her son that she still loved him and that ‘it was the devil that took ahold of you’.

Scales responded that his stepsister’s death was ‘a mistake that happened’.

But his mother told him: ‘That’s not a mistake. You don't mistakenly murder somebody.’

The court heard he had watched his younger sibling every day for months and was never paid to babysit by his mother.

He was sentenced at Montgomery County Court in Dayton after he was found competent to stand trial and changed his insanity plea to guilty.