Father Eugene Lewis was jailed for four years in September 2010.
The 77-year-old was found guilty of 11 charges of indecent assault between 1963 and 1973. He denied the charges.
On Tuesday, senior judges overturned three of the 11 counts and reduced his prison sentence from four years to two years and nine months.
Reasons for the decision reached by the Court of Appeal in Belfast will be given at a later date.
The abuse took place at the County Fermanagh home of the sisters. They said the abuse began when they were as young as seven.
In May 2010, a jury in Omagh unanimously convicted Lewis, who lived in Dublin before his six-week trial.
During the trial, the jury heard that Lewis had befriended the girls’ parents nearly 50 years ago.
Prosecuting QC Ken McMahon said the priest had “wormed his way” into the family and told the jury that although welcomed into their home at anytime, he often chose to call at bedtime or on Saturday bath night.
After the trial, defence solicitor Joe McVeigh said the priest intended to “fight to the last drop of his blood to establish his innocence completely”. [More]