Arthur and Joan Frost, from Prestatyn, Denbighshire, lost their daughter Julie Bennet, and her husband Mike in February.
An image of the couple during their last moments was released by their children.
An appeal has raised over Â£275,00 to help them stay in their home in Wirral.
Mr Bennet, 57, a cabinet maker, had been fighting a brain tumour since 2013. He died on 6 February.
Mrs Bennet, 50, a primary school teacher, died five days later.
She had been diagnosed in May 2016 with cancer which began in the liver and kidneys and then spread to other organs.
Mr Frost said: "The way this went was just horrifying. How can it happen? You don't bury your children, it's just not possible."
"It's not the way it's supposed to be," Mrs Frost added.
"There was nobody like Julie. She was there for everybody. She was absolutely fabulous.
"Michael was the nicest man you could ever meet, he really was. He melted your heart, really."
Mr Frost found out he too had cancer just before Christmas but hid the diagnosis from the family for fear of adding to their strain.
"It was quite devastating to know on top of everything else that had happened," he said. "I'm joining the gang, sort of thing.
"We discussed it at length and said 'there's no way we can say anything under the condition they're in at the moment'."
The family's story prompted a flood of donations to an appeal for the Bennets' children, Luke, 21, Hannah, 18, and 13-year-old Oliver.
Luke has now been granted guardianship of Oliver along with a family friend.
Mrs Frost said the response had been "absolutely amazing", adding, "it restores your faith in human nature".
Mr Frost said: "They've got no income, and we thought well, the mortgage has got to be paid, and that basically was what we were looking for.
"And it just went sky high. It went worldwide. People from abroad have been donating. We just can't get our heads around how wide it's gone."
The grandparents of three children who were orphaned when their parents died of cancer days apart have said they are "horrified" by what has happened.