"A lot of their behaviour started to make sense to me," she told Judge Stephen Clark.
"[My daughter] did her first overdose when 15 and this year has attempted to overdose twice," she said of her once "bright, happy and outgoing child".
She had since withdrawn and relies on medication to cope with daily life.
"I fear for her mental state.
"[McPherson] has ruined [her] life."
Her sister's victim impact statement, read by her aunty, talks of how she doesn't have any male friends as she no longer trusts men.
"I do wonder how my life would have been if I was more social.
"I left school early and I used to be close friends with my sister ... now we interact very rarely.
"I wish our lives were different."
Crown prosecutor Kaleb Whyte said there was a large age gap between both the victims while the offending stretched out over a lengthy period of time.
The offending varied between inappropriate touching and kissing to skin-on-skin contact.
In pushing for a sentencing starting point of four years' jail, Whyte said the victims were vulnerable and added there were no mitigating features for McPherson, saying any good character discount should be tempered by the length of his offending.
McPherson's lawyer, Philip Morgan KC, urged the judge to focus on two key charges of sexual conduct, notwithstanding the number and length of the period relating to the other charges.
Those two charges related to single events, as revealed at trial.
He also defended his client's otherwise good character.
"Despite the fact that this man, as a consequence of these convictions, can be regarded as somebody who committed sexual offences over a period of time, every other aspect of his life through to today is that he is a man of good character.
"He does appear here for sentence essentially for two serious offences that happened at a different stage of his life, 10 years ago."
He added that a pre-sentence report had identified McPherson as being at low risk of reoffending.
He urged Judge Clark to adopt a starting point of 30 months and after giving a good character discount, could arrive at an end sentence in the range of home detention - 24 months.
That report recommended community detention and supervision, a term that Judge Clark said he disagreed with.
"I understand your employer speaks highly of you and there's no real issues about drug and alcohol use.
"You continue to maintain your innocence which is your right and consistent with your approach at trial."
He said there was "no doubt" his offending had impacted the victims and while he adopted part of Morgan's sentencing rationale, there were still a number of aggravating features including that the more minor offending continued for years.
Judge Clark took a starting point of 34 months before issuing a six-month discount for his previous good character and being at low risk of reoffending.
McPherson, who was supported in court by family, will now automatically be put on the Child Sex Offenders register.
As victims of sexual assault, the sisters have automatic name suppression.
