In the preview clip released by the Australian show, Thomas Markle Jr said "I told Prince Harry I think she's gonna ruin your life. She's very shallow."
He also added: "I'm Meghan Markle's brother, I'm the biggest brother of them all."
Mr Markle Jr has reportedly been estranged from his sister for almost a decade but said he was "baffled" when he wasn't invited to her sister's wedding in 2018, according to the MailOnline.
He said at the time: "I’m not bitter, just baffled. It’s hurtful given how close we once were. I’m confused and a little distraught because here is a person knowing the position she’s in and knowing the scrutiny she’s under and yet she has forgotten her flesh and blood."
The duchess's brother has previously attacked his sister in the press calling her a "bully" and a "sensitive young woman" in an interview with German newspaper Bild.
Mr Markle Jr is the son of Meghan's father, who also has a difficult relationship with his daughter.
This season of Australian Big Brother has already courted controversy when producers invited British far-right political commentator Katie Hopkins on the show.
Ms Hopkins was ditched by the programme after she boasted about flouting Australia's tight quarantine restrictions for foreign arrivals.
Her social media posts, showing her breaking the rules, sparked anger in the country which prompted the government to deport her.
Mr Markle Jr will be joined by former Olympic gold medallist and American media personality Caitlyn Jenner who is currently running for the 2021 governor recall election in California.
Former advisor to Donald Trump Omarosa Manigault Newman, who became critical of the president and released a book about her time in the White House, will also be taking part in the Australian show.
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