Elizabeth Mulcahy, of Llandaff, Cardiff, has been charged with six counts of indecent assault against a female under 13.
The offences are alleged to have happened in the 1970s and 1980s.
Cardiff Crown Court heard the incidents left the girl feeling "scared" and "dirty".
The court heard Mrs Mulcahy first touched the girl inappropriately when she was under 10.
"I was too frightened to tell anyone," the woman said in the video of her initial statement to police, which was played to jurors.
"I feel responsible because I feel like I should have said something."
She said the sexual abuse stopped when she was 13 after she started to "show resistance".
The trial continues.
An 82-year-old woman accused of sexually abusing a young girl left her alleged victim "frightened to sleep at night", a court has heard.