Police have said they will not be able to question the 28-year-old, who had emergency surgery to save his life, until next week.
Neighbours said the relatives often had loud arguments and there is some suggestion they were arguing over an inheritance.
On Sunday morning, at around 11:30, the aunt and nephew were heard rowing inside their home and shortly afterwards she is believed to have fired a revolver, which she owned legally, at her nephew.
Two bullets hit a door and one hit the 28-year-old in the back. He was able to snatch the revolver from her, run out of the house and throw the gun in a nearby meadow.
However, the 83-year-old followed him, found the weapon, went back into the house and locked herself in. Then she is believed to have shot herself.
The Special Task Force Cobra was alerted and broke down the door to the house. They found the pensioner, covered in blood, sitting at the kitchen table.
She was admitted to the state psychiatric hospital Wagner-Jauregg, where she later succumbed to her severe head injuries. Her nephew was taken to hospital in Linz.