Another man, James "Jim" Scott Church, 51, has pleaded not guilty to murdering Leisl Smith, who vanished from the NSW Central Coast on August 19, 2012.
The body of the 23-year-old has never been found.
After police obtained CCTV footage of Church giving Ms Smith a lift at Tuggerah railway station on August 19, he said he had dropped her off at Wyong.
In the NSW Supreme Court on Tuesday, Grace Smith gave evidence about Craig Elkin who was her sister's boyfriend some years before her disappearance.
The Crown alleges Church killed Ms Smith after she told people she was pregnant to him and because he wanted to save his new relationship with Belinda Lees.
But the defence contends that Craig Elkins, who died in 2015, cannot be ruled out as the killer.
While Ms Smith said she did not see much of her sister in the year before her disappearance, she had lived with her and Mr Elkins some time earlier.
She said he was violent towards her sister many times, leaving bruises on her, and once red marks around her throat where he had grabbed her and once a welt when he smashed a full coke can on her head.
"She loved him but was scared of him and wished he didn't treat her like that."
She said Mr Elkins said if her sister became pregnant to another man, "He would kill her and him, whoever the guy was".
She herself was regularly threatened by him and an AVO was taken out against him.
Once he threatened to kill her when he accused her of being the source of a rumour that he had touched one of the girls he coached at soccer.
In the year after her sister disappeared, Ms Smith said she happened to be walking across a road and Mr Elkins was coming in the other direction.
She tried to avoid looking at him and starting speaking to someone, but Mr Elkins yelled out: "What the f*** are you saying about me, you slut?".
Ms Smith said she replied: "What did you do to my sister, tell me what you did to her?".
He replied that she should ask their father and Jim Church what they did to Leisl.
When he was with her sister, Mr Elkins tried to make himself look like "a big man" by saying he knew The Bra Boys and other gangs.
He used to say "you are going to be a dead dog" and once smashed a statue of a dog and left the head there like the horse's head in the Mafia movie.
He'd told her he had killed someone and threw the body over a bridge and used to talk "about cutting people up and throwing them to the pigs".
"He used to steal his threats from the movies," she said.
She had come to realise he had been "all talk, no bark".
Ms Smith said while she was told her sister and Mr Elkins re-ignited their relationship before she disappeared, she didn't see them together.
Her sister never told her Jim Church had ever been violent towards her.
"I never heard a bad word about him," she said.
The judge-alone trial, before Justice Elizabeth Fullerton, continues.
1800 RESPECT (1800 737 732)
9°C