Thomas Schreiber was convicted on Friday following a three-week trial at Winchester Crown Court of stabbing the pair at the baronet’s country estate near Gillingham, Dorset.
The judge, Mr Justice Garnham, told the 35-year-old defendant: “The only sentence I can pass is of life imprisonment but for the offence of murder I have to set the minimum number of years and I also have to sentence you for the attempted murder of your mother.”
Mr Schreiber admitted carrying out the “horror show” knife attack on the pair on April 7 2021 – the eighth anniversary of the death of his alcoholic father David Schreiber – after he said his mother Anne Schreiber shouted at him for being “drunk like his father”.
He told the court: “I just went completely crazy, I completely snapped and I just launched for her and began attacking her uncontrollably.”
He had admitted manslaughter but denied the murder of the baronet, saying he had “lost control” of himself and had not intended to hurt him or his mother.
Ms Schreiber suffered up to 15 separate injuries on her head and body, including a partially severed spinal cord which has left her paralysed.
Sir Richard suffered three deep wounds to his face and five to his chest, up to 12cm deep.
Schreiber had also pleaded guilty to driving a Range Rover dangerously on the A303, A4 and M3 when he failed to stop for armed police in a 135mph pursuit into central London.
As he was detained in Chiswick, the defendant stabbed himself in the chest and told officers: “Please kill me now, please just shoot me.”
The trial has heard the defendant was angry with his mother for “abandoning” his father, who suffered from depression, to move in with Sir Richard.
Schreiber, an aspiring painter, also felt Sir Richard treated him unfairly compared to his two sisters despite him receiving a £1,000 monthly allowance and £100,000 towards buying a house.
The court heard family arguments descended into violence on three previous occasions, including once when Sir Richard hit the defendant with his walking stick in November 2020. It caused the cane to shatter and left Schreiber “humiliated”.
The defendant blamed lockdown for worsening his mental health by forcing him to remain in the “toxic pressure boiler” environment of Sir Richard’s estate.