The shock horror case, made public by the Herald Sun, involved a man accused of fathering four children with his daughter, whom he held as a virtual prisoner, raping her almost daily from when she was 11 years old.
A brother of the alleged victim said the family had their suspicions over the drowning of their brother at a suburban Melbourne beach more than 30 years ago, the Australian newspaper reported.
The woman also apparently told friends that she thought her sibling died under mysterious circumstances.
"I don't like talking about it because I can still remember him," News.com.au quoted her brother as saying.
"I wish he grew up, but let's just say there were a lot of suspicious things about it. It was a long time ago, I don't know what the police think about it," he added.
Police decline to say if the death would be investigated.
A spokeswoman said, "Due to the amount of information provided, our avenues of inquiries are limited and we are unable to establish confirmation of the allegations."
She added, "Due to the current suppression order in existence it would be inappropriate to provide any information surrounding this."