The group of friends had been drinking at Sims' flat when things took a turn for the worse, the court heard.
She became angry after talking with the victim, who was 17 at the time.
"I told her how her baby's dad used to talk to me and I didn't know they were still together ... I didn't do anything with him ... but she got mad," the victim told the court yesterday.
Sims said she wanted to fight the other girl, Roper took her glasses off her and told her to fight back.
But the teen did not fight back, she simply tried to protect her face as the blows kept coming.
She told the court there were too many blows to count from both Sims and Roper.
Sims then used bottles to sexually assault the teen.
All three defence lawyers claim their clients, Roper, Kerr, and Olsen were not in the backyard when the sexual assault took place. They suggested that the teen could not see who was watching the assault because her glasses had been taken off her and she had her hands in front of her face shielding herself from the blows.
But the teen was adamant she could see Roper and heard Kerr and Olsen laughing when she defecated because of the attack.
"They were laughing about it," she told the court.
It was also suggested that the four bourbons she had drunk that night along with the knocks to her head could have confused her memory. Again she denied this.
Lawyer Tony Snell, suggested the teen wanted his client, Kerr, to "go down" for the incident because he did nothing to stop it happening. She insisted she was telling the truth.
Other people at the flat that night were called to give evidence and said the teen was shaking with fright and wouldn't look anyone in the eye.
Neighbours came over to the flat and asked to see the girl they had been told had been beaten up.
Sims and Roper told the victim to put clothes on and walked her out to the front of the house.
She was shaking and looked frightened a witness said but declined the neighbour's offer to take her home.
A witness who had passed out from too much alcohol told the court he saw the victim the next morning looking bruised, her hair had been cut and she looked "pretty shaken up". He called his mother and she came and picked the pair up.
The trial continues today.