Maureen Cochrane, 45, and her 54-year-old husband Alex died after the blaze broke out at their home in Brooklands, Greater Manchester.
Their 16-year-old daughter Lucy, who has learning difficulties, remains critically ill in hospital.
A 39-year-old man and three women, aged 65, 39 and 17 - all from the same family - are still being questioned on suspicion of murder.
Yesterday, the Cochranes’ 25-year-old son David, who lives in London, revealed his sister had been bullied at school and blamed the arson attack on “sheer victimisation”.
He said the family had been “let down” by police, who failed to investigate a previous incident when petrol was poured on the Cochranes’ doorstep.
Detective Superintendent John Dineen, from Greater Manchester Police, apologised to the family and said the matter had been voluntarily referred to the Independent Police Complaints Commission (IPCC).
The fatal attack took place in the early hours of Thursday last week at the family’s house in Warmley Road. Mrs Cochrane, a craft shop worker, died at the scene. Her husband, a baggage handler at Manchester Airport, lost his fight for life three days later in hospital. Lucy, a special needs pupil at Manchester College in Northenden, remains seriously ill in Wythenshawe Hospital, unaware that her parents and pet dog Buster are dead.
Yesterday David Cochrane said Lucy had been bullied during her time at Newall Green High School in Wythenshawe and said he was convinced that the bullies were behind the arson attack.
He said: “These people couldn’t deal with the fact my sister was a very happy girl who didn’t let things get her down.
“She was bullied at school, she was very badly beaten up and verbally abused. She took to hiding in the toilets at school because she has special needs and she thought this was the only place she could go.”
Mr Cochrane denied reports that his family had been involved in a feud.
He said: “This was completely one-sided, it was sheer victimisation for being slightly different.”
His uncle Mike Williams added: “It was because their daughter was expelled from school, they targeted the family afterwards. I put it down to that, they weren’t pleased with it at all.”