Gina Rinehart

More dirty laundry for Gina Rinehart, left, as emails are revealed. Photo: Andrew Meares

Gina Rinehart's family feud has stepped up a notch, with youngest daughter Ginia releasing private emails sent by her brother John to their mother in which he described Ginia as "incompetent, craving attention and superficial" with a brain like ''a weak bowl of porridge''.

John Hancock, 38, sent one of the emails in February last year, just days after a suppression order was lifted and the family's mammoth dispute was aired.

He told his mother that Ginia, then 25, could not help run the company because "you have spoiled her so much her brain has turned into a weak bowl of porridge'', according to an affidavit tendered by her lawyer, John Dalzell.

The bitter dispute erupted days before Ginia was to turn 25 the previous September, a trigger for vesting of a $5 billion trust set up by their grandfather Lang Hancock.

The net worth of Mrs Rinehart, Australia's richest person, this week fell $3 billion to $16.95 billion in a slump at Hancock Prospecting.

In one email, sent by John to his mother, he accused her of running a "campaign the whole way to keep me down'' and chastised his sister, calling her soft.

"And why shouldn't she be after how you have mollycoddled her! A Rolls at 25. Really mother, Silly on your and her fronts."

When email extracts were released on Wednesday, Mr Hancock admitted he had "criticised Ginia's conduct, harshly, in … emails addressed to my mother only''.

He questioned why Ginia told her lawyers to release the emails, saying the action confirmed his view mother and daughter were working together behind the scenes.

The case will return to court next year.