One of the alleged victims, now 46, claimed that she had screamed at Paul Greig, 51, to leave her sister alone, and had pleaded with him to take her instead.
"I was her big sister. He just carried on. He just laughed," she told a jury.
The younger sister described how she had tried to hide behind a couch to escape Greig's attentions. And when she heard that he insisted nothing had ever happened, she stated: "He is lying."
Greig, of Armadale, West Lothian, an officer with Lothian and Borders Police, denies indecent behaviour towards the girls and raping them on various occasions between 1974 and 1975, when he was 14.
The older sister told the High Court in Edinburgh that she remembered being nine or ten when Greig had come to their home to babysit a few times.
"The whole night was really spent abusing myself and my sister, having sex with us, moving from one to another. Every time he babysat, it followed the same course," the woman said.
She recalled one occasion when Greig was having sex with her sister. She had screamed at him to stop and to do it to her instead, but he laughed.
"He used to threaten us our dad would be put in jail and we would be put in a home if we ever told anybody," the woman added.
She had told no-one until later life, and it was only because her sister had been "going through a really difficult time" last year that she had said to her that she would support her and come forward if that was what she wanted.
"I did not want to make a complaint to the police. I always found a reason not to. In my heart of hearts, I did not want to come forward. To me, it was buried at the back of my mind," she said.
In her evidence, the younger sister said she had been about seven when Greig babysat. She had not liked him and he frightened her.
She alleged that on one occasion he took off her pyjamas in the living room, and he lay on top of her and raped her on the couch.
"I could hardly breathe. I could hear my sister screaming... 'leave her'," the woman told the court.
However, no official complaint had been made against Greig.
In 2007, the woman saw a photograph of Greig in a newspaper in relation to a story about a missing man. He had been the policeman holding a poster giving the man's details.
The defence counsel, Tommy Ross, put it to the witness that in statements, her parents said they had never seen anything untoward after Greig babysat.
"That's because I was too scared to say," she replied.