Charges were laid only in 2020, when the defendant's daughter came forward with sexual allegations.
Police contacted the man's niece to see whether she would change her mind about giving evidence against her attacker.
Despite her ostracism from the family, she agreed to.
The first incident, some time between August 2011 and June 2012, occurred when there was a family gathering at the house where the victim was living.
After spending the night drinking outside, the defendant sneaked into an impromptu camp the children had set up.
He put his arm over his daughter and groped the victim over her clothes, which resulted in her fleeing to her own bedroom.
Later, when the teen was staying at the defendant's house, the man again slid into a bed being shared by the two girls and repeated the molestation.
"The offending stopped not because you came to your senses but because the victim removed herself from the situation," Judge Michael Turner said.
The victim said being shunned by the family made her feel "all alone in the world".
She described the process of giving evidence at trial as "brutal" but said there was some comfort in the fact the jury had believed her.
Judge Turner said the young victim was especially vulnerable because of her age, the familial link and the fact the incidents happened in places she could have expected to feel safe.
"It could be described as largely opportunistic but it was also brazen given the presence of others including your own daughter and wife," he said.
Though the defendant had no previous sex convictions, it was about the only type of crime lacking from his list, the judge said.
His "extensive" history, spanning 40 years, included theft, burglary, receiving, unlawfully taking vehicles, aggravated robbery, weapons charges, assault on police, domestic violence, breaches of protection order, breaches of sentence and drink-driving, the court heard.
The defendant had spent a large portion of his childhood in state care.
While that did not provide a direct link to his offending, Judge Turner accepted he had a disadvantaged upbringing, being victim of and witness to bullying and abuse.
The defendant was assessed as a high risk of reoffending and was placed on the Child Sex Offender Register.
The jury cleared him of sexual offending against his daughter.