The man, who cannot be named, has been on trial in the ACT Supreme Court from late last week, facing 13 offences against the three children.

The siblings have all given evidence against their former stepfather, alleging he abused them in the early 1990s at a home in Palmerston.

The Crown says the children were too scared to say anything about their stepfather, who was allegedly violent toward their mother.

On one occasion, at their home in Victoria, the father allegedly dragged their mother into their room by her hair late at night, and told the children to look at their "slut of a mother".

Much of the alleged sexual abuse is said to have taken place in the bedroom of the children's mother and stepfather.

The Crown says one incident saw the two sisters forced into performing indecent acts on the man at the same time.

But the defence, represented by barrister Richard Thomas, say the allegations are untrue.

Mr Thomas said the girl's mother had sold her home to invest in the stepfather's ailing flower business, but had never seen any money in return after they split in 1993.

He has accused the siblings of making the allegations up.

The younger sister has questioned why she would do such a thing.

"Who would think up these things?" she said on Tuesday.

"I have no interest in making up allegations against someone else."

The jury began their deliberations in the trial on Thursday afternoon.

They will continue deliberating on Friday morning. Acting Justice Stephen Walmsley is presiding over the trial.