However she continued to deny the charges, claiming the children were lying and she "did not believe in physical discipline," Justice Gilbert noted.
Failelei had adopted three of her niece's children in October 2005, and they had lived with her on and off between 2006 and 2014.
She was charged with physically abusing the two older sisters over several years using a broom stick, a jandal, parts of a vacuum cleaner and the handle of a mop as well as punching, kicking and smacking them.
One charge related to the whole period one victim, known only as J, lived with Failelei between the ages of 11 and 18.
"The offending involved you giving J hidings using part of a vacuum cleaner or the handle of a mop if J was disobedient or answered you back. J sustained bruises as a result of these assaults," said Justice Gilbert.
The remaining charges all related to offending against the other sister, E, when E was aged between 11 and 13.
"These offences covered five separate incidents involving hitting E with a broom stick across her shoulder and legs, punching her, smacking her with a jandal, smacking her
on her face with your hands, and kicking her.
"E suffered bruising and on one occasion, a bleeding nose."
Justice Gilbert ruled the main factors contributing to the offences were Failelei's propensity to use violence and "poor relationships [she] had formed with marriage partners".
A community-based sentence was considered appropriate, he said, in part because of Failelei's ongoing contact with her daughter's six children, age six to 23.
On each charge, Justice Gilbert sentenced Failelei to six months' community detention, 40 hours' community work and six months' supervision, to be served concurrently.
She was ordered to remain at her home at all times during a curfew period between 7.00pm and 6.00am until her sentence was completed.
Failelei was also ordered to undergo anger management counselling and was banned from contacting all three of her adopted children.
She also had to be supervised by an adult, approved by the probation officer, when in the company of children.