The rift between American actress Meghan Markle and her half-sister Samantha Grant seems to be escalating by the day. After branding her sibling – who is rumoured to make her engagement with Prince Harry official – as unfit for royalty, the 51-year-old has now threatened to spill family secrets that Meghan "won't like".

Markle's estranged sister is set to make the revelations in a tell-all book, which will contain her memoirs and some "personal" details about her bi-racial family. Even the working title of the book The Diary Of Princess Pushy's Sister, seems to take a dig at Prince Harry's girlfriend.

"My book deals with my bi-racial family in a candid, warm, personal and socially important way," Samantha, who is Markle's sister from her father Thomas Markle's first marriage, told the Daily Star. She added a word of caution for her actress sister.

"Maybe when Meghan is more mature and reads the book she'll understand. Some of it she won't like, some of it she might," the former actress/model added.

This is not the first time that Samantha has targeted her sister.

Soon after the royal romance received an official nod last October, she branded Markle as "narcissistic and selfish". The Florida-based writer revealed that she raised her sister for 12 years, but once Markle tasted success, she never "mentioned" her.

"The Royal Family would be appalled by what she's done to her own family," Samantha said about her sister. "The truth would kill her relationship with Prince Harry, he wouldn't want to date her anymore 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."

The bitter family feud came in the limelight after the Suits actress declared herself as the "luckiest girl in the world", following the announcement of her romance.

Unfortunately, her sister had a different take on the budding romance and had told The Sun: "Hollywood has changed her. I think her ambition is to become a princess. It was something she dreamed of as a girl when we watched the royals on TV."

Markle, who is also a humanitarian and businesswoman, is rumoured to tie the knot with Harry later this year.