James Jahn dated Marli from February 2014 and was in a relationship with her when her father, mother and older brother were slain in an axe attack in their home at the upmarket estate, de Zalze, in Stellenbosch.
Marli, who was 16 years old at the time, survived the attack. She suffered a brain injury and has retrograde amnesia, but has since made a physical recovery.
Her brother Henri van Breda was arrested for the murders more than a year after the January 27, 2015, killings.
Jahn told the court that he knew Marli's parents well and that they socialised with his parents.
He saw the family about three times a week and saw Marli every day at Somerset College where they attended school.
He said he would describe Henri "almost as a good friend", and said they did a lot together.
He visited their house often, and when they went on holiday he would join them. "As a normal family, they all seemed to have good relationships with one another. A few arguments arose here and there as all families do."
He told the court that Marli loved her brothers very much. She looked up to them for "inspiration and academic choice".
Her relationship with each of them was different. "She would talk to Rudi about academics and to Henri about music and sport."
He testified that the parents, Martin and Teresa van Breda, had a "good interaction" with their children, and often wanted to spend time together as a family.
Jahn said the parents had a good relationship with each other, although the father was more dominant, "a more controlling figure in the family".
He told the court that Teresa wanted a little more freedom after returning to South Africa, and Martin wasn't used to that.
Martin's relationship with all three children was good he added. "He dealt with the children in a similar way, he wanted the best for his children and wanted them to be diligent and work as hard as possible. He tried to be a good father."
He recalled an argument that the family had, that Marli told him about on January 10, 2015.
She had apparently been criticised by the family about her weight gain, a subject she was very sensitive about.
The family had also wanted Marli to scale back her relationship with Jahn as they felt it was interfering in her school work.
Jahn said Marli had been very upset after the argument, and had told him her mother had been too.
He then wrote Marli a message he described as a "mistake".
It read: "Right now I feel like I want to murder the people that are around you at the moment ... But I am being strong to help you and support you babe xxxx Pls I can call you whenever you need me. Don't let me sit in the dark like this babe".
During cross-examination, he told the court that the family wanted the teenagers to focus on schoolwork.
"I was concerned about why Theresa was so upset, something else may have cropped up."
He said he saw the family the following day and they all went surfing together.
Jahn said police had asked him to explain the Whatsapp message. He believed they wanted to rule him out as a suspect.
His cellphone had been confiscated by police two days after the murder.
Henri van Breda appeared emotional for the second day in a row, occasionally wiping away tears as Jahn described the family dynamics.