A High Court judge was in tears as he spoke of the "immense burden" suffered by the family of Becky Watts as he jailed her two killers.
Mr Justice Dingemans sentenced Nathan Matthews, 28, to a minimum of 33 years in jail for murdering his 16-year-old stepsister.
Matthews' girlfriend, Shauna Hoare, 21, will serve 17 years imprisonment for the manslaughter of Becky.
The pair targeted Becky due to their shared dislike of her and their sexual interest in petite teenage girls.
They took tape, handcuffs and at least one stun gun to her home in Crown Hill, Bristol on February 19 and let themselves in when they knew she was alone.
Detectives say the truth about what actually happened to Becky when she was smothered to death may never be known.
The popular teenager – dubbed "Bristol's angel" by her family – suffered more than 40 injuries to her body as she bravely fought for life against her porn-obsessed stepbrother.
Former TA soldier Matthews, along with Hoare, dismembered Becky with an £80 circular saw in the bathroom of their squalid home in Cotton Mill Lane.
They callously laughed as they watched a parody of a Disney Frozen song, entitled Do You Want To Hide A Body, while Becky's remains lay in their bath.
Both Matthews and Hoare were convicted of conspiracy to kidnap, perverting the course of justice, preventing Becky's lawful burial and possessing two stun guns.
A jury of 10 women and one man – eight of whom returned to see the pair sentenced – took just three hours 27 minutes to reject their lies.
The judge was clearly emotional, with tears in his eyes, as he rushed from court, having paid tribute to Becky's family.
"The evidence proves, I am sure, that Nathan Matthews had developed a fixation with having sex with petite teenage girls, and Shauna Hoare had been persuaded to participate in this fixation," he said.
"Becky could be described as a petite teenage girl.
"It is apparent that neither Nathan Matthews or Shauna Hoare have truthfully said what happened at 18 Crown Hill to Becky and I understand the family's sense of frustration about that fact."
Becky's family and friends, wearing blue ribbons in her memory, crowded into the upstairs and downstairs public gallery of court room two at Bristol Crown Court.
They heard how pornography found on a laptop and phones owned by Matthews and Hoare revealed a shared sexual interest in teenage girls and threesomes.
Sinister Facebook messages in which the couple discussed kidnapping schoolgirls for sex revealed the motive behind Becky's death, the judge said.
"I am sure, on the evidence that was adduced at the trial, that the planned kidnap of Becky was for a sexual purpose," he added.
"The false suggestion given by both of them during the trial, namely that these texts were just banter or pandering to tastes, shows how much their understanding of where proper boundaries of conduct began and ended had become corrupted and warped."
Matthews will be 61 before he can be considered for parole, with the judge noting that he "might never be released".
The court heard that Matthews quibbled about the price of the circular saw he used to dismember his stepsister, which he bought the day after her murder.
As he began the process of cutting Becky's body into eight pieces, Hoare texted him "lol" as she sat with his worried mother Anjie Galsworthy - Becky's stepmother.
Matthews and Hoare told police and Becky's family a web of lies, insisting they had heard her leave home on February 19.
"Their deceit of the family was particularly cruel and unusual and the family's sense of betrayal by Nathan Matthews and Shauna Hoare is both understandable and justified," the judge said.
"The dismemberment of Becky's body has been fully described in the evidence. There are features which defy any reasonable explanation."
The judge issued commendations to police who "relentlessly" searched for Becky while members of her family and community "hoped and prayed that she was still alive".
"Finally I should like to pay public tribute to the family of Becky for the dignified way in which they have conducted themselves throughout these proceedings," he said.
Welling up, pausing, and his voice appearing to break, the judge added: "Hearing the evidence during the trial has been difficult for anyone but it is plain that it has been an immense burden for the family."
Harrowing victim impact statements were read to the court on behalf of Becky's father, Darren Galsworthy, and her mother Tanya Watts.
Mr Galsworthy said: "Police informed us that body parts had been found and then we were told who did it.
"These family members sat in our home knowing what they had done and watched my very public descent into madness and despair.
"They said nothing and carried on with the pretence of helping us and showed no emotion at all."
Ms Watts spoke of seeing her daughter in the mortuary, with bandages wrapped around her body to hide where she had been dismembered.
"I often hear myself talking about Becky and asking questions about her death but it is like I am talking about someone else, not my baby girl. It is surreal.
"But the actual reality is those people who were involved in Becky's murder, dismemberment and concealment have left us with a lifetime of emptiness, continuing nightmares of her final moments and a grave to visit."
Karl Demetrius, 30, and Jaydene Parsons, 23, who admitted assisting Matthews to hide Becky's body parts in their shed, will be sentenced in February.