Nathan Matthews’s girlfriend Shauna Hoare was cleared of her murder but found guilty of manslaughter.
Matthews, 28, suffocated his 16-year-old stepsister while trying to kidnap her from her semi-detached home in Crown Hill, Bristol, on February 19.
He dragged her body into the boot of his Vauxhall Zafira and drove it to the home he shared with girlfriend Hoare, 21, less than two miles away in Cotton Mill Lane.
Over the next three days, Matthews bought an £80 MacAllister circular saw, goggles, masks, bags, cling film and tape to cut up and package Becky’s body in the bathroom.
Packages containing eight body parts were stored in suitcases and a box then moved into a shed in Barton Court - just 80 metres away - in the early hours of February 24.
By that time, Avon and Somerset Police had launched the force’s largest missing person’s investigation, which culminated in Becky’s remains being discovered on March 3.
A jury of 10 women and one man at Bristol Crown Court returned verdicts after just three hours and 27 minutes of deliberations on six charges against Matthews, Hoare, and two men accused of helping to move and hide Becky’s body parts.
Matthews was found guilty of Becky’s murder, while Hoare was acquitted of the charge but convicted of her manslaughter.
Both were found guilty of plotting together to kidnap 5ft 1ins Becky in a sexually motivated plot fuelled by a shared desire for petite teenage girls.
Hoare was also convicted of perverting the course of justice and preventing Becky’s lawful burial by dismembering and hiding her body.
James Ireland, 23, and Donovan Demetrius, 30, were both acquitted of assisting an offender.
Friends and family of the popular teenager, including best friend Courtney Bicker, grandfather John Galsworthy, uncle Sam Galsworthy, brother Daniel Watts, stepmother Anjie - Matthews’s mother - father Darren Galsworthy and aunt Sarah Broom, sat in the public gallery wearing blue ribbons.
As the first verdict was returned - that Matthews kidnapped his stepsister with Hoare - people in the upstairs public gallery began clapping and shouting “Yes”.
Matthews and Hoare remained totally emotionless as all the verdicts were returned. Trial judge Mr Justice Dingemans said he would pass sentence tomorrow from 10am.