"We'll all have a hole in our hearts forever thanks to this callous attack," she said in a statement.
Ms Landreth said her brother had loved the outdoors.
"Now he can't even open his eyes to see the sun," she said.
"He can't do anything but lie there, probably wanting to die and be at peace."
Landreth was driving to a party with friends when he stopped at the service station in Hillside Rd to buy energy drinks.
Rakete - wearing his Mongrel Mob patch - arrived minutes later to fill his BMW.
As the victim returned to his car, the pair exchanged words.
Rakete punched Landreth, his head smashed on the concrete and he lay there unconscious.
The gang member simply left him and continued fuelling up his car.
When the attendant went to call emergency services, Rakete tried to grab the phone from him.
"Nah, *** that guy, just leave him," he said.
A truck driver, and former nurse, saw Landreth prone on the forecourt with blood coming from his ears, pooling around his head.
She asked Rakete whether he had hit him.
"What the **** it got to do with you lady? ****** stay out of it," he responded.
When she pressed him further he admitted he had.
Rakete added: "he should've shut his ****** mouth."
As Landreth lay convulsing and gasping for air, the defendant took one of the drinks the victim had bought and poured it on his face.
Rakete left at speed.
Ms Landreth described the harrowing scene when she arrived at Dunedin Hospital.
"I saw my brother lying there with tubes coming out of every orifice," she said.
"Scars and stitches and blood."
Landreth had suffered an infection recently which resulted in him again being rushed to intensive care.
"For 48 hours we didn't know if he would live or die," his sister said.
"And it's still the same - no-one knows what his fate will be ... It could be like this forever."
Just months before the petrol station incident, Rakete had been walking on Rattray St with friends and spat in the face of a driver who refused him a ride home.
He admitted that assault too.
Judge Macdonald said the defendant had a history of violence and previous stints behind bars had done nothing to change his tendencies.
"This is a graphic illustration of the very tragic consequences that can happen when punching someone in the head," he said.
"You could easily have been facing a charge of manslaughter."
Rakete gestured at supporters as he was led to the cells.