Popular dinner lady Elizabeth Edwards, 49, and her youngest daughter Katie, 13, were found butchered at their home in Spalding, Lincs., on April 15 this year.
The jury has now retired to consider a verdict on the 15-year-old girl who admits manslaughter, but denies murdering the pair who were stabbed and smothered to death.
Nottingham Crown Court heard the “sweethearts” – aged 14 at the time – stabbed both victims in the throat before having sex together and watching vampire film Twilight.
The court also heard how the girl wrote “help me” and “death is the only way” in her diary before the murders took place.
Addressing the jury on what would have been Katie”s 14th birthday, Mr Joyce said: “You have heard just how determined these two children were to take two lives.
“You know what they planned, how they planned it and you know the awful detail of how they carried it out.
“You heard her say: “He continually asked me do you want to go through with it, and I said yes”.
“She only had to say no and it would never have happened.
“Even after Ms Edward”s death, even after she”s seen her dead, seen her struggle, heard her gurgle, she still gets [the boy] to kill Katie instead of her – to make her gurgle too.
“The sheer brutality and utter contempt oozes from ever pore from this girl in those interviews.
“You cannot ignore the sheer coldness and callousness of the killings.
“They say they were supposed to commit suicide on Thursday at 2pm. “First of all we”ll get the alcohol in, then I”ll take the pill, we”ll have sex, we”ll watch Twilight”.
“What kept you? This was no suicide plan. Actions speak louder than words.
“She had been “excited” and “looking forward” to killing.”
Andrew Stubbs, QC, defending the girl, urged jurors to look at psychiatric evidence which showed she was suffering from an adjustment disorder at the time.
He said: “This 14-year-old girl at the time is responsible for the killings, let”s be clear on that.
“You have heard the interviews. Could there ever have been a more clear and open acceptance of responsibility.
“Why? Why was she so clear? The answer to that may well help you answer the questions that you have to answer.
“What this trial has been about is not who is responsible for these killings, but to what extent is she responsible.
“From the outset she could not have made the prosecution”s case to prove what happened any easier.
“She could have sat in the police interview and said nothing.
“She was a 14-year-old girl confessing to her part in these terrible tragedies, and you are left with the stark alternatives presented by Dr Joseph and Dr Chakrabarti.”
Adjustment disorder is the name given to a group of symptoms seen in those struggling to cope, including stress or feeling sad after a stressful event.