Samantha Markle, also known as Samantha Grant, claimed the prince should have married ex-girlfriend Cressida Bonas instead because she’s “much more like Diana”.
In a series of tweets, she then hit out the Duchess of Sussex and accused her of faking a “ridiculous” British accent.
Mr Grant, who is writing a book titled ‘In the shadows of the Duchess’, wrote: “Harry is a wuss to allow the Duchess of Nonsense to mistreat everyone who has been close to her, especially her family.
“Diana would be ashamed. Stop wearing Meg's skivvies and bra and take your pants off of her!”
The Duchess has not spoken to her half-sister for years and did not invite her to the royal wedding in May.
Ms Grant, 53, has regularly attacked her online and in interviews, accusing her of ignoring their father Thomas and the rest of their family.
She took the blame for her 73-year-old father Thomas Markle’s staged photos with the press just days before the royal wedding.
She said she had suggested he do “positive photos for his benefit and the benefit of the royal family”.
Ms Grant also wrote on Sunday: “Sorry tough love exists and just because I can speak out about some things does not mean I don’t love her but I can call out things I do not agree with... if nobody likes it it’s too damn bad.”
Her book was initially set to be titled ‘The diary of Princess Pushy’s sister’ but now appears to have been changed.
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