The child, who cannot be identified, voiced their fears after brothers Blake and Tristan Barrass were killed by their parents.
Blake, aged 14 and Tristan, 13, were strangled and suffocated in their home on Gregg House Road, Shiregreen, in May.
Their mum planned to kill the boys, her four other children and then herself because of fears the children were at risk of being taken into care, Sheffield Crown Court heard yesterday.
Barrass, 35, enlisted the help of her 39-year-old half brother Brandon Machin, who was also the father of her six children, to help her carry out her plan.
The children had believed Machin was their uncle.
The pair were both jailed for life yesterday and ordered to serve a minimum of 35 years behind bars for their crimes.
The two oldest surviving children – who the incestuous couple plied with prescription drugs in an attempt to kill them before trying to drown one of them in a bath - have been left ‘emotionally broken’, Sheffield Crown Court heard.
And all four remaining children will be in need of ongoing support, according to a victim impact statement written on their behalf by social workers.
The court heard that the children wanted their parents to go to prison for ‘300 years’.
Kama Melly QC, prosecuting, told the court: "When (the older two children) were told Sarah and Brandon had pleaded guilty to the murders of their brothers and the attempted murders of them, (one of them) said they were worried they would become a murderer when they were older because that's what their mum and Brandon did. They said they didn't want to be like that."
The court heard that the children did not know that Machin was their father and had been told their father was dead.
Miss Melly said: "Both (the older children) are emotionally broken and don't know why this happened. They repeatedly ask why and how. We don't have the answers."
The barrister added: "Both (the older children) keep saying they just want a nice family home."
She added: "Both say they want their brothers back because it's too hard without them."
The court heard that the two youngest children, who are aged under three, never ask for their mother or Machin, even when they are upset.
But Miss Melly said the older two were ‘really struggling knowing they will not see their big brothers again and not seeing their (other siblings) every day’.
She added: "They have lost everything."
"There's no doubt that all four children will need ongoing psychological support.
"There's no way of knowing the long-term effect and impact on their lives at this stage."
Mr Justice Goss, who jailed Barrass and Machin, said: "Both the children who survived were clearly aware of some of the terrifying events surrounding the deaths of their brothers and your attempt to drown one of them."
He continued: "The initial effect on them, as described by them, was frightening. Unsurprisingly they are extremely upset and anxious.
"The statement from their social worker on their behalf describes their inevitable confusion, the effect of the loss of their brothers upon them and the incredible emotional hardship of these events on the (older children) and of being separated.
"Inevitably they will require a significant amount of support. The long-term consequences for them and (the younger children) as they grow older and become aware of what happened cannot be known, but is likely to be significant."