Lisa Borch was 15 when she was committed of committing the crime with her Iraqi boyfriend.
The court was unable to rule which of the pair delivered the 20 stab wounds that killed her mother, Tina Rømer Holtegaard, and both were convicted of murder.
Borch was sentenced to nine years in prison and her 29-year-old boyfriend, Bakhtiar Mohammed Abdullah, was jailed for 13 years, followed by deportation.
But the teenager’s lawyers appealed the term, which Danish media reported to be the longest ever given to a teenager in Denmark, to the Western High Court.
Borch changed her story for the judges, claiming she had falsely admitted involvement because she was “scared of Bakhtiar”, Ekstra Bladet reported.
But the panel did not accept the new account and upheld her previous sentence.
Borch’s twin sister and stepfather were in court for the hearing on Thursday and had previously described how she became obsessed with Isis.
The court examining the case last year heard that she watched the group’s gory beheading videos for several hours before committing the murder and may have been planning to join Isis in Syria with her boyfriend.
The couple killed Mrs Holtegaard as she lay helpless in bed after taking a sleeping pill on 8 October 2014.
Borch called the police herself, claiming she saw a man running from the house, but officers became suspicious after she “shrugged” on being told her mother was lying dead in the bedroom.
Jens Holtegaard, her stepfather, told Ekstra Bladet the teenager started becoming hostile towards her mother months before the murder and that jihadism was not the motive for the murder.
“She might well have been attracted to bikers or any other extreme,” he said. “But I fear that Lisa will be very easy to radicalise in a prison environment.”
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