While the physical touching stopped at this point, her sister told the then-girl, "You should have never allowed this to happen", the woman said.
It wasn't until family matters came to a head several years ago that the woman said she found herself talking to police.
After her mother died the man served her documents relating to a dispute over a familial estate.
The woman told the court she was "disgusted" he had the audacity to serve her papers after she had kept him out of court for four decades.
"I said to him if you want to serve those on me you're telling me we deal with family matters before the courts because there's a matter between you and me that needs sorting out."
She said the man would "engineer" situations to end up alone with her and, not fully comprehending his intentions as a young girl, she would carry on like normal and hope it never happened again.
"He was older than me. He was looked up to within the family because he was educated ... and I just felt he wasn't someone that I could resist; that I had the age [or] the life skills to turn away."
Crown prosecutor Steve Manning began his opening address by telling the jury the fact the incidents were alleged to have happened a long time ago made them "no less important to the woman who alleges today that they happened".
"She simply wants her day in court and wants to be believed," he said.
The man's defence lawyer Roger Philip highlighted the seriousness of the allegations and the fact the woman couldn't be specific in detail when recalling encounters with her brother-in-law.
He told the court to carefully consider all the evidence put forward and proposed the woman had brought the allegations forward in an attempt to get back at her sister.
Under cross-examination the woman said she wasn't motivated by malice, stating "I don't want to be his victim anymore. I want to be a survivor."
The trial is expected to end today.