Paul Cohrs shot dead his brother in NSW and then his 81-year-old mother in Victoria, in October 2018, following a years-long family dispute.
He was on Wednesday imprisoned for a minimum of 23 years in Victoria's Supreme Court over the murder of his mother Bette Schulz Cohrs.
Cohrs' act was premeditated and planned, motivated by anger and grievance, Justice Lesley Taylor said in her sentencing.
"You aimed at an elder lady who was entitled to feel safe in her own home," Justice Taylor said on Wednesday.
"She had been in fear that you would shoot her.
"In her last moments, she would have realised her fears were realised."
The relationship between Cohrs and his brother Raymond became strained in 2012 as they had conflicting ideas over properties connected to the family business.
Raymond, who had the support of his mother, decided to get the properties appraised, including one on the NSW border that Cohrs and his wife were living in.
After turning up at the property's gate with a real estate agent for an appraisal, the brothers exchanged heated words.
All three men drove to a shearing station where Cohrs shot his brother multiple times in the head and chest before handcuffing the agent in the shed.
He then travelled 120km to his mother's home, at Red Cliffs in Victoria's northwest, and shot her in the chest one hour later.
Cohrs drove off, leaving his four-year-old grandson to find Mrs Cohrs' body lying on the kitchen floor covered in blood.
He drove back to Lake Victoria Station, freeing the real estate agent and tried to kill himself.
Cohrs claimed he was not guilty of his mother's murder because he was mentally impaired, but that was rejected by a Supreme Court jury who found him guilty in his trial in May.
He was not tried for the death of his brother in NSW as it occurred in a different jurisdiction.
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