She has told the court that Harris came up to her at the BBC Television Centre in 1983 when she was 13, put his hand under her breast and said "Do you often get molested on a Saturday morning?"
Former TV star Rolf Harris waved to the cameras as he arrived for his child sex abuse trial for the first time since being freed from prison.
Harris denies the allegation and three other counts of groping two more women who were aged 16 or below at the time.
She said Harris' previous conviction had not made it any easier to come forward.
"The court heard the alleged victim made her official complaint in July 2014, the same day as a newspaper reported Harris' victims were to receive "£200,000 in compensation".
Asked if it was said in "a jokey fashion", the woman replied: "No absolutely not".
"She said [Harris] had made her feel really amusing and weird and she didn't like the way he'd made her feel, and she's reiterated that ever since", the sister told the court.
"All I want is finally vindication and justice for the people that this has happened to over the years".
"It's been an very bad experience", she said.
The trial is expected to last the rest of the week.
Harris had so far been attending the trial via video conference from prison.
The woman replied: "I fail to see, whether it is said to a group or a single child, how that can be misconstrued as a joke in any way".
He was asked by prosecutor Jonathan Rees whether he and his wife "cooked this up between you, saw an opportunity, saw pound signs in your eyes" before going after Harris.
It is said he had an "appetite" for inappropriately touching the girls, but Harris says the claims are false and are part of bogus compensation claims.