The boy and girl - both 14 at the time of the murders - planned the killings of 49-year-old Elizabeth Edwards and 13-year-old Katie who were stabbed through the throat and smothered as they slept.
They were jailed for life with a minimum of 20 years.
The pair, now 15, went on to share a bath, have sex, and watch four Twilight vampire films after the murders in Spalding, Lincolnshire, last April, Nottingham Crown Court heard.
Sentencing the pair, who cannot be named for legal reasons, to life with 20-year minimum terms, Mr Justice Haddon-Cave said: "This case is, in many respects, without parallel."
The judge said the case had "defining and particularly chilling" features, including the young age of the defendants and that the offence was carefully planned.
As both teenagers remained composed in the dock, flanked by security guards, the judge condemned their conduct after the killings as "grotesque".
"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," the judge said.
Both the boy and the girl pleaded guilty to manslaughter but not guilty to murder at a hearing in September, with the male defendant admitting murder before the trial began.
His girlfriend maintained her innocence to murder, claiming to be suffering an abnormality of mental function which impaired her ability to form rational judgments, but was found guilty after a five-day trial.
Speaking in April, Mrs Edward's vicar at St Paul's in Spalding said she was "so committed to the life of the church".
Rev Mike Chesher, said: "The church community miss these two people so much and I think we will miss them so more as time goes on because Liz was so committed to the life of the church in so many ways".
