The uncle claimed the baby was injured after he "playfully threw him in the air".
Andrew John Nolan also said Bobby Webber was hurt when the seven-month-old was playing with a mobile phone and dropped it on his face.
Then Nolan said Bobby must have been hurt by an unknown intruder. "I did not do this to Bobby and whoever did this I hope you find them before I do," Nolan told police.
The cabinet maker, then 32, was babysitting his nephew in September 2014 in a house on the NSW Central Coast while the child's parents were away on their honeymoon.
Another relative found Bobby tucked under a sheet in his cot, limp, and sobbing in his sleep, with red marks on his forehead covered in butter, a bite mark on his hand, and redness around his groin.
Nolan said he put butter on the child's head after he hurt himself with a mobile phone, and had given him a dose of the antihistamine Phenergan to settle him.
In fact, Bobby had suffered a fractured skull, brain damage, bruised ribs and limbs, and fractures to both ankles. He also suffered haemorrhages in his eyes, usually seen after cases of violent shaking, or high-impact incidents such as car crashes, or falls from great heights.
The child was left with permanent life-threatening conditions, including epilepsy, cerebral palsy and a movement disorder, and is visually impaired.
Eventually Nolan pleaded guilty to causing grievous bodily harm with intent, but a NSW District Court judge did not accept his account that he simply put his foot on the boy's back, slapped him, and dropped him into his cot..
He was sentenced last year to at least 8½ years' jail, with a maximum of 12½ years.
The Crown appealed against the sentence, arguing the judge should have considered the attack to be in the worst category of offending, and that the sentence was "manifestly inadequate".
On Wednesday, the NSW Court of Criminal Appeal found it was open for the trial judge to find the attack was in the "high range" of offending, rather than the worst case.
But the appeal panel agreed the sentence was not adequate, and added three years to Nolan's non-parole period.
Justice Derek Price said Nolan "intentionally and with considerable force inflicted life threatening injuries upon Bobby, a defenceless seven-month-old".
"The child's prospects of enjoyment of life have been destroyed," Justice Price said.
"The [original] sentence falls well short of adequately reflecting the gravity of the offence, the need for general deterrence and the denunciation of the respondent's conduct.
"Serious physical abuse of infants is abhorred by the community and general deterrence is of great importance in sentencing such offenders."
Justice Price also said: "Public confidence in the justice system would not be served by allowing a manifestly inadequate sentence to stand."
Justice Elizabeth Fullerton and Justice Clifton Hoeben agreed with Justice Price's findings.
Nolan's maximum term is now 15 years and three months. He will be eligible for parole in April 2026.