Ghislaine Maxwell's brother called on US authorities to release his sister from pre-trial detention, insisting she is not a flight risk.
Ian Maxwell said the conditions his sister faced amounted to torture and prevented her from preparing for her trial on sex trafficking charges.
"If she wanted to run away, she could have done so at any point in the year after Epstein died," Mr Maxwell told BBC's Radio 4.
"She is a pre-trial detainee. By definition, that means she is innocent."
"This isn't someone seeking to flee. She has an entire life built over the last 30 years, so she's not in the business of running away from anyone," he said.
Mr Maxwell claimed she would renounce her UK and French citizenships to guarantee bail. These passports matter greatly to her, he said.
"Ghislaine has been in prison now for 250 days and counting," he said. "She is in effective isolation in a cell six foot by nine foot [1.8 metres by 2.7 metres] and a concrete bed. There is no natural light."
"She is under 24-hour round-the-clock surveillance with a camera that tracks her around the room. That is her existence.
"The water that is provided through the prison is brown and the food she is given is highly microwaved and inedible.
"She is resolute, she is strong in her spirt and undaunted. But obviously, this is very, very wearing physically - she a 59-year-old woman.
"We understand she is losing her hair and she's having trouble with her eyesight and her ability to concentrate."
He said it was ridiculous that his sister was considered a suicide risk.
"She's been completely over-managed and that's because Jeffrey Epstein died in federal custody. It's a grotesque overreaction," he said.
Ms Maxwell was arrested seven months ago on charges relating to accusations she recruited three girls for Epstein to abuse in the mid-1990s. She pleaded not guilty.
Epstein, her former boyfriend, died by suicide in a Manhattan federal jail in August 2019 as he awaited trial on sex trafficking charges.