Fifteen-year-olds Kim Edwards and Lucas Markham - believed to be Britain's youngest double murderers - were both convicted last year of killing Edwards' mother, dinner lady Elizabeth Edwards, 49, and little sister Katie, 13.
In interview tapes now released by police, Markham is heard calmly telling detectives how he went to the victims' bedrooms, stabbing and smothering them both, last April.
Markham said: "She (Edwards) always hated her mum and I said 'I wish I could kill her' and she said 'yeah', and thought I was joking - but I was being serious.
"Then she realised I was being serious, so she started being serious."
The victims were stabbed in what the trial judge called "brutal executions" at their home in Spalding, Lincolnshire, last April.
Over the course of the next 36 hours the besotted teenagers, then both 14, had sex, shared a bath and watched Twilight vampire films before police arrived.
Both were detained for life with minimum terms of 20 years last November by a judge at Nottingham Crown Court who said the case had "few parallels in modern criminal history".
A reporting restriction banning identification of the murderers was lifted at the Court of Appeal earlier this month.
The Markham interview featured in a Channel 5 documentary Murdered By My Daughter, which first aired on Monday night.
In it the young killer described to detectives how, as part of the couple's plan, he went to Edwards' home that night with a bag of knives, knocked on his girlfriend's window, and waited as she let him in.
Markham said: "Then I went in to her mum's room, stabbed her in the neck while she was asleep on her side, and smothered her face with a pillow.
"After I knew she had gone I went in to Katie's room - which is the same room as Kim's - and I thought I stabbed her but I'm not 100% sure if it was her or the mattress, and then I smothered her face with the pillow too."
When police asked Markham why the couple decided to kill Katie, he replied: "Because she would call the police."
At their trial in October 2016 Markham admitted murder, while Edwards had denied the charge, claiming she suffered an abnormality of mental function.
However, a jury found her guilty after trial.
In part of her police interview, which has previously been released, she said of the killings: "I was okay with it.
"Just the fact that it happened so quickly, that gave me peace of mind because, you know, it wasn't like torture or anything."
She said the couple carried out the murders because her mother "liked her sister more".
Detective Superintendent Martin Holvey, who led the Lincolnshire Police investigation, said: "You actually recount the story to your team and everyone sits there, hardened detectives, and think, I can't believe what you're hearing."
Speaking in the programme, he said: "You know, over 30 years as a cop, you look at this case and there's nothing that rivals it that I've come across."
Elizabeth Edwards' partner at the time, Graham Green, described young Edwards as "another Myra Hindley" and said she "should be never, ever let out".