Witness Tracy McFetridge, 44, told police she heard Brand make the threat in a conversation with another friend weeks before she was found dead.
The court heard McFetridge made the accusation in a statement she gave to police during a re-investigation in 2020, 24 years after Miss Glachan, who was just 14 at the time, was found dead in the River Leven, near the Black Bridge in Renton.
It is alleged the three accused arranged to meet Caroline at the Black Bridge near a towpath beside the river between Renton and Bonhill in Vale of Leven.
They are then alleged to have assaulted the Notre Dame pupil, shouted and swore at her and repeatedly kicked and punched her on the head and body.
It is claimed the three of them threw bricks at her causing blunt force injuries to her head and body before pushing or causing her to fall into the river, and ultimately murdering her.
O’Brien, Brand and Kelly have all pleaded not guilty and lodged a special defence of alibi.
Ms McFetridge told the court that Caroline had been spending time with O’Brien, but that he had been seeing Brand at the same time and that the older girl was not happy with the situation.
Asked by Advocate Depute Alex Prentice KC if there had been a conversation involving Miss Glachan and a suspected pregnancy, Ms McFetridge replied: “Yes. She said she had missed her period and she might be pregnant.”
She went on to tell the court that Caroline was aware O’Brien was seeing Brand, and that she had advised her not to say anything about the potential pregnancy around her.
“Donna said she was not happy and said she was going to batter her. I told her ‘naw, you canny do that, she is only 14’,” she said.
Questioned by O’Brien’s counsel Ian Duguid KC, Ms McFetridge said she initially did not believe Miss Glachan when she said she might be pregnant, adding: “She told wee white lies.”
She then told the court she accompanied Caroline to be tested for pregnancy, and that it had turned out to be positive.
However, forensic pathologist Dr Marjorie Turner said there was no sign the teenager was pregnant at the post-mortem examination.
Tracy McFetridge added in evidence that Donna Marie Brand “was angry, as you would be if someone was seeing your partner at that age”.
Ms McFetridge added that Caroline had visited her at her father’s house on the night before her death, but this was something she only felt confident in revealing during the re-investigation into her death in 2020.
Asked by Brand’s counsel, Thomas Ross KC, why she had made “absolutely no mention” of the visit in three separate statements she made to police in the three months after the schoolgirl’s death, she said she was “scared” of repercussions.
O’Brien faces a separate charge alleging that on various dates between June 1 1996 and August 25 1996, he assaulted the teenager at locations including Balloch Country Park, Renton, Vale of Leven and “elsewhere in Dunbartonshire”.
Later on Wednesday, a former taxi driver, William Gardner, told the court that he spotted Caroline Glachan shortly after midnight on 25 August 1996.
Mr Gardner, who was picking up a security guard from a warehouse, said he recognised Caroline on Dillichip Loan “straight away”.
He said she was walking at a normal pace and did not look upset, but that he was concerned she was alone and said to the man he had just picked up: “What is she doing down here at this time of night? Her mother would kill her.”
Mr Gardner later saw a hooded figure further down the street, walking about 30 yards behind Caroline.
Mr Gardner said he was wearing a green hooded jacket covering half of his face.
He added the man had a distinctive chin and nose, and may have been wearing the hood as protection against midges or the cold.
The witness also said it was possible it could have been raised to conceal the man’s features.
An E-fit was later produced and distributed to the media in connection with the inquiry into Caroline’s death.
The prosecution put it to Mr Gardner that the person he saw could have been a woman called Alison Curley.
Asked if the image could be Ms Curley, the witness said there was a similarity but he thought the chin of the person he saw was “a lot more pointed.”
Witness quizzed
The trial then heard from Ms Curley, who was with Caroline the night before her death.
She was questioned a number of times by detectives during the investigation.
In a 2021 police interview, Ms Curley said she and a boy had walked with Caroline towards the River Leven.
The witness said she believed she was going to meet Robert O’Brien, who she was said to be in a relationship with at the time.
Ms Curley, 57, claimed the boy with them had asked Caroline if she wanted them to “walk her all the way along”.
The schoolgirl is said to have replied: “No, it is OK, there is Robbie there.”
Defence counsel, Ian Duguid KC asked the witness why this information had not been given during interviews prior to 2021. She said she believed it had been but couldn’t be sure.
The trial before Lord Braid continues today
Top picture: The Black Bridge between Renton and Bonhill, as landmark which has now been demolished.