The 49-year-old man, who cannot be named to protect the identity of his victim, was convicted by a Central Criminal Court jury last May on 56 counts of rape and 15 of indecent assault on dates between December 1983 and December 1986 at the family home and other locations.
The girl was between seven and 12 years old, while her brother was 14 years old when the abuse began.
The victim is now 42 years old.
Ms Justice Eileen Creedon said the abuse was “persistent” with the teenager using every available opportunity to molest his sister.
She noted that he also threatened her, producing their father’s shotgun and telling her he would shoot their mother if she ever disclosed the abuse. She believed these threats
Ms Justice Creedon said she must take into account the man’s age at the time of the offending.
She noted from a report from The Probation Service that he now takes responsibility for abusing his sister.
Justice Creedon took into account the man’s lack of previous convictions, the fact that he had integrated into society well since and was married with three children.
She suspended the final 12 months of the sentence on strict conditions including that he complete a sex offenders programme in custody.
He was registered a sex offender when he was first convicted in May.