Ben Beaumont and Michelle Liddle said they fled as their house north of Brisbane was "burning to the ground".
The blaze was allegedly started by Corey Wardle, the 36-year-old son of Ms Liddle from a previous relationship.
He is in a mental health facility after being charged with arson and attempted murder, a Brisbane court was told on Monday.
In March 2020 the family lost 15-year-old Angus after he was fatally stabbed by two teenagers in a Redcliffe car park north of Brisbane.
Six years later they are again reeling after the suspected arson attack destroyed their Clontarf residence and gardening business.
But they managed to salvage Angus's ashes, kept in an urn in the lounge.
The family said they would be forced to close their gardening business for a couple of weeks as they sourced new equipment and found somewhere to live.
They thanked everyone for their support, with a GoFundMe fundraiser launched by Mr Beaumont's sister Mandy reaching more than $26,000 by Monday afternoon.
"We just managed to get out of the house while the offender was dousing everything and the house was literally burning to the ground," Mr Beaumont and Ms Liddle said in the fundraiser page.
"I think we will need help with some household items shortly and I will post a list of items that anybody could help us with.
"We need to find a home to live in first."
They also felt for their neighbours after their property was damaged in the blaze.
"To my 2 neighbours I hope you are OK. They will both need significant help over the next few months also," they said.
The community is rallying around the family who have received another blow after Angus' murder, among a number of high-profile youth homicides that triggered Queensland's controversial "Adult Crime, Adult Time" reforms.
"My brother's son Angus was murdered in a car park - stabbed to death by two young men who thanks to the laws in QLD are both now out living their lives," Ms Beaumont, an acclaimed author, wrote on the GoFundMe page.
"This destroyed our family back then, and this next blow is yet another bad bad thing.
"No one should ever have to go through this, let unless twice in a lifetime - the violence that has been brought to them is something from the movies."
Wardle was initially charged with arson after being accused of using an accelerant to set fire to the home about 1.50pm on Saturday and fleeing before a police dog squad found him a short time later.
No one was injured but the blaze destroyed the Clontarf house and damaged two nearby homes, prompting neighbours to evacuate.
Detectives later charged Wardle with three counts of attempted murder along with two further counts of arson.
Wardle was set to appear in Brisbane Magistrates Court on Monday.
But the court was told Wardle was being held in a mental health ward and would appear via videolink on Thursday.
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