The Duchess of Sussex's half-brother claims "money and fame" went to the former actress' head "really bad".
Thomas Markle Jr. claims he and Meghan used to be "close" but life in the public eye changed her for the worse and they no longer have a relationship.
Speaking on Australia's Big Brother VIP, he told his housemates: "Last time [I saw her] was at my grandmother's funeral in 2011, and she took off after that and went to Canada. We were close before.
"Money changed her, money and fame went to her head really bad. I guess when you are introduced to the 1 per cent of society - that is most likely the reason that happened to her, when she didn't have anything before."
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Before marrying Prince Harry, the former Suits star was wed to Trevor Engelson from 2011 to 2013 and Thomas slammed the "cold" way she treated the producer.
He said: "The guy she was was married to the first time, Trevor, have you seen those Saw movies? This guy works for that production company. That guy just took care of her. She was adored by him.
"And she just walked all over him and dumped him, that fast. After being married, she sent him the ring in the post - is that cold or what?'"
Neither Thomas Markle Jr nor Samantha Markle were invited to Meghan's wedding to Engelson, despite stating they were "close" to their half-sister.
Thomas warned Harry could be "next" and accused the duchess of already changing the prince as he's never pictured smiling anymore.
He added: "Harry's next. Harry is on the chopping block next. The only difference between now and then, all the photos prior to that he had a smile on his face, all the ones after he doesn't."