Samantha Markle, who formerly went by the surname Grant, hit out at her sister on Twitter as she claimed Meghan had not sent their father a card for his birthday last month.
Writing to her 6,000 followers on her private Twitter account Samantha said: ”Happy Birthday Meg! It would be so lovely and appropriate of you to send DAD a BELATED birthday card for his July 18th @KensingtonRoyal Cheers!”
Samantha is renowned critic of her half-sister Meghan, who she is understood to have not seen in years.
Earlier she took a swipe at Meghan after reports emerged the Duke and Duchess of Sussex would be retreating from the spotlight.
Tagging Kensington Palace’s official Twitter account, Samantha tweeted: ”OK so Cruella Deville is retreating LOL let me know how that works out for you.”
It has been a trying week for Meghan, who today celebrate her birthday at the wedding of one of Harry’s close friends, after her father Thomas Markle gave an explosive interview to the Mail on Sunday.
The former Hollywood lighting director claimed the Royal Family was forcing Meghan to freeze him out as he dragged Harry’s later mother Princess Diana into the row.
Thomas Markle in an explosive interview has claimed he cannot get ahold of Meghan Markle any more as he added: “I’ve just about reached my limit with Meghan and the Royal Family.”
Mr Markle, 74, said: “They have Meghan treating her father in a way that Harry’s mother Princess Diana would have loathed. That’s not what Diana stood for.
“Princess Diana is credited with changing the Royal Family but she wasn’t perfect.
“She was still very much one of them.”
The former recluse, who lives in Mexico, however added that despite the distance he plans to send Meghan a birthday card.
He said: “Just wishing her birthday greetings, nothing more.”
“Meghan is everything to me. I love her and always will.”
A Buckingham Palace spokeswoman previously told Express.co.uk they would not comment on Mr Markle’s remarks.
However, royal expert Richard Fitzwilliams believes Mr Markle’s latest comments would have further infuriated the Windsors.
Mr Fitzwilliams told Express.co.uk: “Thomas Markle should have known that such a contentious and highly personal interview would be anathema to the Royal Family.
“His comments about what Princess Diana would have thought will be particularly contentious.
“The Royal Family have changed a good deal since the tragic death of Diana 21 years ago.
“It is clear that Mr Markle’s remarks will not endear him to Princess Diana’s sons, Prince Harry and Prince William.”
Today Meghan was pictured attending the wedding for Prince Harry’s close friend Charlie van Straubenzee to filmmaker Daisy Jenks.
Meghan Markle looked stunning in a dark shirt and multi-coloured skirt combination and matching dark fascinator as she arrived at St Mary the Virgin Church in Frensham, Surrey.
Mr van Straubenzee, 30, was an usher at Harry and Meghan’s Royal Wedding in May, along with his older brother Thomas van Straubenzee.
Princess Eugenie and fiance Jack Brooksbank, who wed in the second Royal Wedding of the year, were also in attendance at the lavish ceremony.