Cheryl Gene Grimmer was abducted on January 12, 1970, outside a shower block at Fairy Meadow Beach in Wollongong, on the NSW south coast.
She was enjoying a day at the beach with her mother and three brothers.
A coroner in 2011 ruled that Cheryl was very likely dead, but her body has never been found and the cause and manner of her death remain undetermined.
Cheryl's is one of a number of long-term missing and murdered cases being examined in a NSW Parliamentary Inquiry, which resumes public hearings on Friday.
One of her brothers, Paul Grimmer, said his sister's disappearance and the lack of answers around it had left his family broken.
"As I got older I felt inadequate to stop my parents from suffering," he said in a written submission to the inquiry ahead of his scheduled appearance.
Mr Grimmer was scathing of NSW Police in his submission, saying they failed to keep the family informed of updates to the case throughout the years.
He called for reform to the way police liaise with bereaved families, including the establishment of a peak body to represent the loved ones of long-term missing persons and victims of unsolved homicides.
"No family should feel how my family was made to feel," Mr Grimmer said.
"If police had shown empathy in the beginning when Cheryl disappeared and updated my family, my parents may have been a little less broken."
NSW Police, acting on fresh information, conducted a new search for Cheryl in bushland in the Wollongong suburb of Balgownie last year.
But testing revealed the bones found and photographed were those of an animal.
Former NSW police assistant commissioner John Laycock, who worked as a criminal investigator for 22 of his 37 years in the force, is also set to front the inquiry on Friday.
In his written submission, Mr Laycock - who retired in 2004 - acknowledged there were "serious deficiencies" in the force's response to historical unsolved homicide investigations.
But he slammed the inquiry, which is chaired by Legalise Cannabis Party MP Jeremy Buckingham, citing comments from current homicide squad head Detective Superintendent Joe Doueihi that it would divert resources away from where they were needed.
"Here we have a section of politicians who have become instant experts in homicide investigations … delving into an area completely above their heads," Mr Laycock wrote.
"It should be disbanded at the first opportunity."
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