The victim told the jury that she then went to bed and fell asleep but then woke to Williams putting his hand over her mouth, telling her that if she said anything he would fire her father.
He then went on to have sex with her.
She said nothing for more than 40 years. It was only at an unveiling when she heard her younger sister mention how Williams had indecently assaulted her that she decided to go to police.
The younger sister testified that she also knew Williams was her father's supervisor.
She recalled how she was in bed and felt someone tugging at her blankets. She initially thought it was a family member but then recognised Williams who tried to kiss her.
He then put his hand under the blankets and fondled her private parts, telling her to keep quiet or his father would lose his job. She told one of her sisters but then remained quiet until the unveiling in 2004.
Williams' lawyer Sacha Nepi said her client admitted having sex with the 16-year-old but said it was consensual. He denied touching the sister.
She said her client was not a regular visitor at the home and he could only remember being at the house on one occasion.
He denied threatening the sisters with their father's loss of job.
Nepi said her client had a clean record and spent many years living in Papua New Guinea where he had been given the British Empire Medal for his services to the community in helping fundraise for the local rugby league and Masonic lodge.
She pleaded with the judge to take into account his age and ailing health and hand down a sentence that was "not so crushing on this elderly man".
Judge McGuire told Williams that children were entitled to feel safe in their home and in their own beds, but "they were not safe on these two occasions".
"They were at the time vulnerable on account of their age and the offending was made worse by threats that you made on each occasion."
In her victim impact statement, the oldest victim said over time she had become anxious and depressed.
"She was unable to get out of bed and my personal care suffered because I couldn't get out of bed so I wouldn't take care of myself."
However, she said her scars would heal and she would eventually "walk with her head held high".
The younger victim said she was robbed of her carefree, happy childhood and when turning into an adult would be forced to take leave from work after suffering heart palpitations from stress.
Judge McGuire agreed with Crown prosecutor Louella Dunn's submission of a six year starting point. He gave Williams a 33 per cent credit for his previous clean record, age and health and sentenced him to four years' prison.
"Their own home was the one place where they were entitled to feel safe and you took that safety away from them."
