A couple accused of murdering Becky Watts had threesomes with a female friend, a court has heard.
Nathan Matthews, 28, and Shauna Hoare, 21, had consensual sex with the woman on at least two occasions - with Matthews filming on his mobile phone.
The pair, alleged to share an interest in petite teenage girls, are accused of killing Matthews' stepsister Becky, 16, in a sexually-motived kidnap plot on February 19.
Bristol Crown Court heard they dismembered 5ft 1in Becky into eight parts, which were discovered in a garden shed 80 metres from their Bristol home on March 3.
Jurors heard Matthews and Hoare, who have been in a relationship for six years, had threesomes with a woman in Bristol in early 2014.
"It started as a joke when I was with my partner," the woman, in her early 20s, said.
"Because I spent a lot of time with them, he joked and said 'you are always there, are you having a threesome?'
"We laughed it off because it was ridiculous. By 2014 it came up in general conversation."
The witness was shaking and crying as she gave evidence while Hoare coughed and wept in the dock.
She went on holiday to Butlins with Hoare, Matthews, Becky, her friend, her stepmother Anjie Galsworthy and father Darren Galsworthy in August 2014.
"There was banter between all of them, including Nathan and Becky," she said. "They just seemed normal, just a normal family."
However, Matthews told her he "hated" his stepsister due to her treatment of his mother, Mrs Galsworthy, and described her as a "brat".
"Whenever her name was brought up he got agitated," she said, adding that Hoare also disliked Becky.
She described Matthews as "paranoid", "jealous" and "controlling" of Hoare, including what she ate.
"He wouldn't let her have energy drinks because she was supposed to be losing weight," she said.
Sarah Hainsworth, professor of material and forensic engineering at the University of Leicester, gave detailed evidence about dismemberment.
Mr Galsworthy and his father, John Galsworthy, sat in the public gallery as a 1400w Mac Allister circular saw was operated.
It was identical to the £80 saw Matthews bought on February 20, the day after killing Becky at her home in Crown Hill, Bristol.
Prof Hainsworth, describing the saw as "powerful" and "very noisy", said there was no direct evidence to suggest how many people had dismembered Becky with it.
"However, it would be easier to conduct a dismemberment such as the one that I have seen if more than one person was involved," she added.
A lack of "false cuts" - areas that were not fully sawed through - suggest Becky's dismemberment was "very careful and methodical".
Claire Morse, a forensic scientist, matched blood staining on door frames at Becky's family home to the teenager's DNA using her toothbrush.
Safety goggles found in a Eurohike rucksack in the shed had a mixed DNA profile, which Becky and Matthews may have contributed to.
One dust mask had DNA from Becky and Matthews on it while another had Becky's blood and "all the (DNA) components of Mr Matthews and Miss Hoare", she said.
A patterned size 8-10 onesie and green Boohoo jumper - thought to have been worn by Becky when she died - had been cut with scissors, she said.
The knot of a bin liner had a mixed DNA sample which could have been contributed to by Matthews, Hoare and Becky, Ms Morse added.
A bag containing Becky's body parts had DNA Hoare could have contributed to, the court heard.
Forensic teams spent days examining Matthews and Hoare's bathroom but found no trace of Becky's blood.
"I find it extremely surprising that we found no blood in that bathroom," Ms Morse added.
She said dismemberment could have happened if all surfaces of the bathroom, the walls and bath were covered.
Matthews, of Hazelbury Drive, Warmley, South Gloucestershire, denies murder and conspiracy to kidnap.
He admits killing Becky, perverting the course of justice, preventing the burial of a corpse and possessing a prohibited weapon.
Hoare, of Cotton Mill Lane, Bristol, denies murder, conspiracy to kidnap, perverting the course of justice, preventing burial of a corpse and possessing a prohibited weapon.
The residents of the Barton Court property, Karl Demetrius, 30, and his partner Jaydene Parsons, 23, admit assisting an offender.
Donovan Demetrius - Karl's twin brother - of Marsh Lane, Bristol, and James Ireland, 23, of Richmond Villas, Avonmouth, a work colleague of Karl, deny the charge.
The trial continues.