In a police interview played in court she said the 16-year-old "knew how to work people".
Becky's stepbrother Nathan Matthews, 28, and his girlfriend, Shauna Hoare, 21, deny murder.
Earlier the jury visited the house in Bristol where it is alleged the teenager was killed by her stepbrother.
The ten women and two men spent 16 minutes inside the house at Crown Hill, in the St George area of the city.
They also visited another property where her body was allegedly dismembered, and the garden where her body parts were discovered.
In the police interview, recorded before Ms Hoare was arrested, she told detectives Becky had recently "bonded" with her stepbrother who had been helping her to buy a mobile phone.
"Nathan had this thing that if he heard her coming down the stairs he would hide behind the door and jump out at her and she jumped every time," she said.
The prosecution alleges Becky was killed at the family home in Crown Hill on 19 February.
Her body was taken to Mr Matthews' and Ms Hoare's home in Cotton Mill Lane where it was allegedly dismembered.
Body parts were later found at an address in Barton Court.
Mr Matthews of Hazelbury Drive in Warmley, South Gloucestershire has admitted manslaughter but denies murder and conspiracy to kidnap.
He has pleaded guilty to perverting the course of justice, preventing burial of a corpse and possessing a prohibited weapon.
Ms Hoare of Cotton Mill Lane in Bristol, denies murder, conspiracy to kidnap, perverting the course of justice, preventing burial of a corpse and possessing a prohibited weapon.
Two other men on trial, Donovan Demetrius, 29, and James Ireland, 23, deny a charge of assisting an offender.
Karl Demetrius, 29, and Jaydene Parsons, 23, previously pleaded guilty to assisting an offender after body parts were found in their shed, but both said they were unaware of what the packages contained.
The trial at Bristol Crown Court continues.

Stephen Ngila has been arrested for allegedly attacking Jackline Mwende almost two weeks ago, reports say.
Kenya has high levels of domestic abuse but Mrs Mwende's case was "particularly shocking", activists added.
Mr Ngila has not yet commented.
Mrs Mwende, 27, said a drunk Mr Ngila, 34, told her "today is your last day" before he attacked her at her parents' home in a village in southern Machakos County, Kenya's Daily Nation newspaper reports.
This was despite the fact that doctors told them she was fertile and he was impotent, Mrs Mwende was quoted as saying.
A video of her with stitches on her head and stumps where her hands once were, wrapped in bandages, has also been shown by Kenyan media.
The couple were living separately when Mr Ngila allegedly attacked Mrs Mwende, reports said.
Reuters news agency quotes Kenyan women's rights group Equality Now as saying that Mr Ngila has been charged with attempted murder, but there has been no independent confirmation of this.
"This is a particularly shocking case for Kenya... even though domestic violence is rampant," Naitore Nyamu of Equality Now told Reuters.
"We can't afford to let the perpetrators act with impunity - which has sometimes been the case in recent years," she added.
The chairwoman of Kenya's Federation of Women Lawyers (FIDA), Josephine Mong'are, said cases of violence against women had been "escalating" and police should ensure that "all culprits are brought to book".
"FIDA Kenya reiterates that violence against women is unacceptable and must not be condoned in society," she said.
The woman accused of murdering Becky Watts told police the teenager was good at manipulating people and was an attention seeker.
[NEXT_CONCEPT]
A Kenyan man accused of chopping off his wife's hands and hacking her head with a machete after blaming her for their childless marriage should face the full force of the law, rights activists have said.