The 51-year-old has the same father as the television star. Thomas Markle, it can be revealed, was declared bankrupt only weeks ago. Once one of Hollywood's most successful lighting directors, he filed for bankruptcy with debts of $30,000 (£24,600), in what appears to have been a dramatic fall from grace.
The 72-year-old has since decamped to Mexico without leaving a forwarding address or telephone number.
Meghan's mother, Doria, who divorced Mr Markle when their daughter was six, also separately declared bankruptcy several years ago with credit card debts of $52,750.
Mr Markle has another son, from before his marriage to Doria, who has also recently declared himself bankrupt.
Miss Markle's extraordinary family background emerged as she was busy declaring herself "the luckiest girl in the world".
Miss Grant, a writer from Florida, said she had brought up the actress for 12 years when she lived in California and declared: "The Royal Family would be appalled by what she's done to her own family. The truth would kill her relationship with Prince Harry, he wouldn't want to date her any more because it puts her in a bad public light.
"I certainly don't think Harry or his family would want to associate with her afterwards."
According to a report on the US website Radar Online, both Miss Grant and her father have been hurt by Miss Markle's 'lack of emotional and financial support' since she became famous.
The website also claimed that the actress was "overwhelmed" by the interest in her blossoming royal romance and is "barely sleeping". An insider was quoted as saying: "She is desperate for the relationship to work."
None of this, however, was apparently stopping the ambitious Miss Markle from publicising her new clothing range, which goes on sale today, or her glossy "life and style" website.
In an interview, she told a Vancouver newspaper: "My cup runneth over and I'm the luckiest girl in the world!"
Describing her style as "aspirational girl-next-door", she also admitted that she was a "brash American" who liked to have her say, but said her mother, a yoga instructor, always cautioned her to dress less sexily as she should "never give the milk away for free".
Miss Grant also revealed that her half-sister has always wanted to be a princess and that she has a penchant for red-headed men.
"Hollywood has changed her. I think her ambition is to become a princess," she told the Sun.
"It was something she dreamed of as a girl when we watched the royals on TV.
"She always preferred Harry - she has a soft spot for gingers."
Despite refusing to comment on the stories about her royal romance, there was still time for Miss Markle to post a picture of a cup of tea and a jigsaw to publicise her website, The Tig, on the picture-sharing site Instagram.
She also uploaded an enigmatic shot of two bananas cuddling with the words "sleep tight".
The post prompted a flurry of comments from her 1million followers, who referred to her as "Princess Meghan Markle', and asked "which one is Prince Harry?"
Miss Markle's Instagram feed also boasts several teasing posts, including one from a visit to London in early July when she is believed to have met Harry for the first time, which showed a picture of a sweet bearing the caption "KISS ME" and the words "Lovehearts in London".
She divorced US film producer Trevor Engelson after a marriage that lasted just two years and recently split from the Canadian celebrity chef Cory Vitiello. According to sources,
Miss Markle has been dating Harry for two to three months after they met in London and bonded over their shared passion for humanitarian work.
