Jonathan Gallagher was only nine years old when he collapsed during a swimming lesson and died.
Nobody was able to tell his family why it happened and a post-mortem examination was inconclusive.
His mother, Blanche, and father John, accepted the findings at the time but 23 years later tragedy struck when their youngest daughter, Lauren (13), also died from an unexplained heart condition.
Her family have been too devastated to speak in the past but have decided to tell their story as part of an attempt to promote heart screening of young people – a process they believe has saved the life of at least one other member of the family.
Mrs Gallagher, from Strabane in Co Tyrone, was heavily pregnant in September 1985 when she received the devastating news about her son.
"He had taken three fainting spells – one while we were on holiday, a wee one at school and then that day," she recalled.
"I left him off to school as usual and went to work. It was just after the summer holidays and he was going into the pool to learn how to swim.
"It transpired that once his body hit the water, he died. They had not idea what was wrong with him.
"When John died, I just thought it was natural causes and probably dealt with it by looking after Rachael, who was born four weeks later."
Mrs Gallagher went on to have another daughter, Lauren, the youngest member of the family. She too, suffered from fainting spells but the doctors believed they were not life-threatening.
"One day she had a faint and I knew it just wasn't like the rest. She recovered at home and was fine but I wrote a letter to her doctor the next day because I was petrified.
"He rang me three weeks later to say he wanted to send her to a specialist in Birmingham but I said to him: "There's no need to do that because Lauren died last night."
Lauren died on February 22nd 2006 as the family sat around in their kitchen having tea.
"It was six o'clock and she just took a faint at the breakfast bar. Everyone did everything they could but I don't know whether it was being her mum or because of the last fainting spell but I just knew."
It is understood Lauren died from abnormal heart rhythms.
Mrs Gallagher said she believed families were often too scared to consider screening children.
"It is so frightening because anyone looking at Lauren would know she didn't look sick. She wasn't sick," she said.
"She was never off school a day in her life."
She said she found it incredibly painful to talk about her children's deaths but she believes a screening programme may have helped save the life of her grand-daughter, Nicole, who was also discovered to have a heart abnormality.
"That is the redeeming feature in all of this," she said.
"My grand-daughter was five months older than my daughter so she's always a constant reminder.
"But it is in a nice way a reminder, I suppose, of what could have been."
