The youngest person to be found guilty of murder in the US is set to be released at the age of 29.

Curtis Jones was 12 when he and his 13-year-old sister Catherine shot and killed their father's girlfriend Sonya Speights in 1999, Florida Today reports .

The pair were initially thought to have killed Ms Speights out of jealousy, but the siblings later claimed they had been sexually abused by a male relative.

After an alleged encounter in the shower with the relative, Jones wrote in her diary she was "gonna kill everybody".

The pair planned to kill their alleged abuser, Ms Speights and their father with a pistol their father kept in his bedroom.

But after shooting Ms Speights dead, they fled into nearby woodland until being arrested by police.

The siblings became the youngest children in US history to be tried with first-degree murder, which was reduced to second-degree murder through a plea deal.

They were sentenced to eighteen years in prison and before being transferred to his cell, Curtis Jones asked his lawyer if he could bring his Nintendo video game to prison with him.

Their alleged abuser, who was convicted in 1993 of molesting his girlfriend's daughter, was never charged or identified publicly.

Now 29, Curtis Jones will leave prison having never lived by himself and with little knowledge of the outside world.

His sister, who he has not seen for nine years, is due to be released next month.

Both will be on probation for life, with conditions so strict they cannot travel without notifying their parole officer.

Unlike her brother Catherine Jones has spoken to the media during her imprisonment and speculated whether they were better off having grown up in prison, away from a dysfunctional and allegedly abusive home.

"There are some times when I look back at it and wonder if this didn't happen, would we have healed to the point where we are?" she said earlier this year.