The first abuse was perpetrated when one of the girls was between four 4 and 5 years old.
While sentencing him, Judge Keith de Ridder said that on one occasion the man took exception to the behaviour of one of his victims and made her stand in front of the TV for so long that she ended up urinating on the floor.
The man then rubbed her face in the urine before taking her to the shower, where he assaulted her.
Judge de Ridder cited a gross breach of trust. He said the girls were vulnerable, given the man's predatory behaviour.
He said the man's own upbringing was extremely fraught but that did not in any way justify or mitigate his offending.
Defence lawyer Chris Muston objected to the Crown's application for a minimum non-parole period but Judge de Ridder said the issues of accountability, denunciation, deterrence and protection of the public were not met by not imposing such a sentence.
He ordered the man to serve eight years in jail before he was eligible for parole.
It emerged at the trial that the man began raping the elder sibling when she was 11 years old and stopped only when she was about 17. He told her not to tell anyone about the sexual abuse because it was "her and papa's secret".