By Lucy Adams
BBC Scotland social affairs correspondent
A joint Fatal Accident Inquiry is to be held into the deaths of Katie Allan and William Lindsay who both took their own lives in Polmont Young Offenders Institution, the BBC understands.
Katie, who was 21, and William, 16, died in separate incidents within months of each other in 2018.
The long-awaited inquiry is to be announced later this week.
Their families said they were frustrated they had to wait five years for an inquiry.
They said that after so many years they were still waiting for answers.
The BBC understands the FAI is due to begin in January next year and will run for six weeks.
Holding a joint FAI is not common but it is not unprecedented.
Katie took her own life at Polmont in June 2018 while serving a sentence for a hit-and-run crash while drunk.
William’s death was in October 2018 while he was on remand at the prison near Falkirk.
Their suicides followed a series of deaths in Scotland’s prisons, which prompted a Scottish government review into how deaths in jail are handled.
John Reilly, William’s brother, told the BBC: “The amount of time it has taken has been the hardest part for me and my family.
“In the past five years we have lost so much.
“My mum and two sisters all died with broken hearts after William died without having clear answers. I just hope now we are getting somewhere and the correct actions are taken by the people responsible.”
Mr Reilly added: “Both William and Katie have been neglected by a system that should have been there to help and protect their needs.”
William Lindsay – Died October 2018
William spent most of his life in the care system. With drugs and domestic abuse at home, he was placed on the child protection register as an infant.
He lived between family, foster carers, children’s homes and secure housing units.
As his mental health deteriorated as a teenager, schools struggled to cope with his behaviour. He was often in trouble with the police and he attempted suicide several times.
In the months before his death, his behaviour improved. He returned to live with his mother and was removed from the Vulnerable Young Persons’ register.
On 2 October 2018, he was detained after walking into Saracen police station in Glasgow with a knife. His motivation was unknown.
The Scotsman reported that social workers and the Scottish Children’s Reporter Administration wanted William to remain in the children’s hearing system and be placed in a secure unit – but there were no places available.
He was remanded to Polmont, where he took his own life three days later.
Katie Allan – Died June 2018
Katie, a geography student at Glasgow University, was jailed for 16 months for drink-driving after hitting a 15-year-old boy, who was knocked unconscious.
The incident happened in Giffnock, East Renfrewshire, in August 2017. Katie had drunk four pints of beer before trying to drive home from a pub.
She pleaded guilty at Paisley Sheriff Court to causing serious injury by driving dangerously and driving at more than four times above the legal alcohol limit.
Her parents Stuart and Linda claimed bullying and “humiliating” strip searches led her to take her own life at Polmont
They said prison staff failed to heed warnings that Katie was “vulnerable” and had a history of self-harming.