One of the surviving children of Sarah Barrass and Brandon Machin fears they could go on to kill "because that's what mum and Brandon did".
Barrass and Machin, who is her half-brother, strangled Tristan and Blake Barrass, aged 13 and 14, in Shiregreen, Sheffield, in May.
They had both previously admitted murder, conspiracy to murder all six of their children, including Blake and Tristan, and five counts of attempted murder.
All the surviving children, who cannot be named for legal reasons, are under the age of 13.
The court heard the children wanted their parents to go to prison for "300 years".
The two older, surviving children - one of whom they tried to drown in a bath - have been left "emotionally broken" by the actions of their parents, jurors heard.
In a victim impact statement, the court was told one of them feared becoming a murderer.
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."