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.
The woman accused of murdering Becky Watts told police the teenager was good at manipulating people and was an attention seeker.