Thomas Markle Jr - aged 51, from Grants Pass, Oregon - has written a handwritten letter to Harry, urging him not to marry his sister saying: "It's not to late".
Thomas Markle Junior, who shares a father with the former Suits actress, told the 33-year-old royal it is "not too late" to call off the May 19 union.
Thomas Markle Jr also claimed his father had not been invited to Prince Harry and Meghan Markle's wedding, despite other sources reporting that he would be in attendance.
"I thought, 'This can't be real, '" she said.
The letter goes on for two pages, with the 36-year-old bringing their father, Thomas Markle Snr, into it and claiming Meghan "used" him to start her career before leaving him alone and bankrupt in Mexico.
Thomas, who lives in OR in the U.S., went on to say that once Meghan made it big in Hollywood she changed and forgot anyone who was in her life before.
"'It's very apparent that her tiny bit of Hollywood fame has gone to her head".
Harry also remains socially connected to both women, with Bonas remaining close friends with his cousin, Princess Eugenie, who has her own wedding later this year.
Kensington Palace says the couple hope it "will likely be a memorable second for everybody who has gathered collectively in Windsor to benefit from the ambiance of this big day".
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Although it may seem odd to invite your exes to your wedding day, there is a royal precedent.
'She has a large family.
"She is acting phoney".
He further added, "Meghan Markle is obviously not the right woman for you".
"Prince Harry, you are a really lucky guy to have her", he said at the time. "She's getting in there and it's the family I suppose that she's never had".
"But I guess we're all distant family to Meg".
Mr Markle accused his sibling of turning her back on the family and said she is "falsely" trying to portray herself as the new Princess Diana.
However, the author does admit "there were some sort of flirtations while he was with Chelsy but actually, his and Chelsy's relationship was pretty up and down and Chelsy was just as much a flirt I think as Harry was".