Gina Rinehart's son John Hancock privately called his youngest sister Ginia "intellectually disabled" and told his mother she had spoilt the 28-year-old so much "her brain has turned into a weak bowl of porridge".
Allegations of intense animosity between the estranged siblings appear in fresh documents the NSW Supreme Court released yesterday. They detail the depth of the rift which has split the famous family.
In them, Ginia's lawyers accuse John of bearing an unusual degree of spite towards the youngest Rinehart, calling her "incompetent, craving attention and superficial" in communications with his mother and Hancock Prospecting executives.
He also said she and her sisters were "useless oxygen thieves".
At one point, he suggested his mother tell Ginia to wear a bag over her head and that "she can't help that you have spoilt her so much her brain has turned into a weak bowl of porridge".
Of the four children, he said Ginia was most under the mining magnate's influence.
Ginia's release of the heated emails between John and his mother comes as the youngest Rinehart escalates her attack on the suitability of her sister Bianca to head the family trust at the heart of the dispute.
The sides are deadlocked over leadership of the trust, which owns almost a quarter of family company Hancock Prospecting.
Bianca is the only family member to put her name forward to be trustee but Ginia - the only one of Mrs Rinehart's four children to stand with her mother throughout the two-year legal battle - opposes her bid.
Ginia said she feared Bianca, who sides with John, shared her brother's animosity towards her.
The new claims come two weeks after Ginia's lawyers moved to get access to Bianca's medical records, including documents "constituting or evidencing any counselling, therapy, psychiatric or psychological treatment" Bianca had received.
Bianca, who later revealed she once struggled with an eating disorder, described this as a "personal attack".
Mr Hancock condemned the use of his personal emails in the move against Bianca, saying his comments about Ginia were made privately to his mother.
He said he criticised Ginia's conduct harshly in private emails addressed to his mother only but would never have made them public. "Ginia has decided to do so," he said.
A Supreme Court hearing into the trust's future ended two months ago with NSW Supreme Court Justice Paul Brereton asking all parties to agree on a suitable replacement for Mrs Rinehart, who stepped down as trustee days before a trial into her fitness to hold the position.
Her other daughter, Hope Welker, was part of the action against her mother but withdrew this year amid financial trouble.
The case will return to the court in February.