"Punches were thrown," Judge Michael Turner said.
Macahan was marched out of the house by his sister and a friend and they drove off.
During the drive, the defendant became agitated.
His sister slapped him across the face in a bid to bring him to his senses, the court was told.
But it did not have the desired effect.
Macahan, who was sitting in the back of the car, punched his sister in the face, "yelling and swearing" at her.
She was hit in the face five or six times, a Crown summary said.
Lacey Macahan got into the back and tried to hold her brother's arms to stop the onslaught but he kept punching her.
With the victim closer to him, Macahan also headbutted her and told her: "You're dead, you're dead".
Eventually she succumbed to the blows and ended up in the footwell in the back of the vehicle as he continued throwing punches.
Macahan then began to strangle his sister.
"She was defenceless and lost consciousness, thinking she was going to die," Judge Turner said.
The attack only stopped when the driver pulled over and intervened, with the help of those in a car following them.
When the police arrived, Lacey Macahan had several seizures. It was unclear whether they were a result of the attack, the judge said.
She was left with bruises to her face, neck, arms and hands and two black eyes.
Defence counsel John Westgate said his client previously suffered a serious head injury in a car crash and was now committed to staying away from alcohol.
The twins had attended a restorative justice conference, where Macahan's apology had been accepted by his sister.
In an interview with Probation, the defendant told them he remembered going to the Dunedin party on his birthday but nothing more.
"You only realised something was wrong when you woke up in police cells the following morning," Judge Turner said.
Macahan was sentenced to nine months' home detention, 300 hours' community work and ordered to pay his sister $500 as emotional-harm reparation.
