Image caption Elizabeth Edwards, 49, and her daughter Katie, 13, were found dead in Spalding in April

Two 15-year-olds have been given life sentences with 20-year minimum terms for the murder of a mother and daughter in Lincolnshire in April.

Dinner lady Elizabeth Edwards, 49, and Katie, 13, were smothered and stabbed while they slept at home in Spalding.

Sentencing them, the judge said it was "a terrible crime which has few parallels in modern criminal history".

The killers were 14 at the time, apparently making them Britain's youngest double-murderers.

The judge at Nottingham Crown Court, Mr Justice Haddon-Cave, said the case had "defining and particularly chilling" features, and condemned the killers' conduct as "grotesque".

Jurors heard that the killers went on to share a bath, have sex, and watch four Twilight vampire films after the murders in Dawson Avenue, Spalding, on 13 April.

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Image copyright Police issue Image caption The knife used in the murders - a black-handled kitchen knife with a 20cm blade - was found discarded in Katie's bedroom

Addressing the teenagers, the judge told them: "There is a clear intention to kill both victims - both defendants admitted wanting to murder them.

"There was remarkable premeditation and planning - it was, on any view, substantial, meticulous and repeated."

He added: "The killings were brutal in the form of executions, and both victims, particularly Elizabeth Edwards, must have suffered terribly in the last minutes of their lives."

Mrs Edwards and her daughter were repeatedly stabbed by the couple, who were described in court as having a Bonnie and Clyde-style relationship

Mrs Edwards was stabbed by the boy eight times, twice in the throat, in a deliberate attempt to ensure her daughter was not woken by screams or cries for help.

Katie was stabbed twice in the neck with severe force before she was smothered by the boy.

Image copyright SWNS Image caption Police described the murder of Elizabeth and Katie Edwards as a horrendous crime

During his sentencing remarks, the judge said both teenagers were equally responsible for the murders and had "revelled in what you achieved".

"I see no reason to distinguish between you in any way," he said. "Had you been adults you may have been facing the whole of your lives in prison for this double murder."

Outside court, Karen Thompson, deputy chief crown prosecutor for the East Midlands, said: "This is one of the most distressing and disturbing cases that I have ever encountered.

"Our deepest sympathies are now with the extended family and friends of Elizabeth and Katie Edwards as they attempt to come to terms with this horrific crime."

Media playback is unsupported on your device Media caption "This has been a rare unprecedented case" - Det Ch Insp Martin Holvey

Det Ch Insp Martin Holvey, of Lincolnshire Police, said it had been "a rare and unprecedented case".

"The planning that went into these brutal murders of Elizabeth and Katie, as they slept in their own beds, was cold, ruthless and chilling - as was the lack of remorse shown by the two juveniles afterwards."

In court Peter Joyce QC, prosecuting, detailed for the first time the account given by the boy shortly after he was arrested.

He said the pair had met up and both agreed to kill Mrs Edwards and her daughter after "the girl had asked him to kill both of the victims as she did not like the smell of blood".

"He went on to describe the killing of the mother, Elizabeth, and said that during the attack she had struggled and scratched his face, back and bum," Mr Joyce told the court.

"After about three minutes she had stopped struggling and had gone limp. He had got off the bed and taken off his trainers since they had made the floorboards creak."

Image copyright PA Image caption Katie Edwards was stabbed to death after her mother

The boy then went into Katie's room, bent down and pushed the knife through her throat before using a pillow to smother her, the court heard.

Defending the boy, Simon Myerson QC said the two children "became trapped in a fantasy of their own devising".

Andrew Stubbs QC, representing the girl, said the pair were in a "toxic relationship" and were "almost playing chicken with each other" as they spurred each other on to commit the killings.

The pair pleaded guilty to manslaughter at a hearing in September but later the boy admitted to murder before the trial began. His girlfriend maintained her innocence to murder but was found guilty of two counts.