Samantha Markle is planning to hijack the birth of her sister's baby with a tell-all book that she claims will expose Meghan's "lies".
The 53-year-old wants to release her book, entitled In The Shadows Of The Duchess, in April or May to coincide with the birth of the royal baby.
According to the Daily Mail she has talked about a memoir for months, previously saying it would be called "The Diary of Princess Pushy's Sister".
But now she claims to have added even more to the book and will 'hold nothing back' on what her sister, 37, is really like.
She told the Daily Star: "In The Shadows Of The Duchess covers everything… the world does not know truth total truth… (I'm) holding nothing back! From lullabies to lies..it's all they're in my book."
Ms Markle, who suffers from multiple sclerosis and uses a wheelchair, is keeping the exact contents of her book a secret.
But she has hinted it will uncover a family feud involving Tyler Dooley, 26, Meghan's brother Thomas Markle Jr's son.
Mr Dooley opened up about the trauma Meghan's new royal status has brought to her family in new MTV series The Royal World saying: 'It's been a really tough time.
"The family has been torn apart. Seeing these vicious headlines about who I am. It's been hard. The struggles in the past year have put me to breaking point."
His aunt Samantha will also talk further about the "cyber bullying" she has experienced since her sister married Prince Harry, 34, in her new book.
She told The Star: "On top of it I am dealing with cyber stalking and cyber bullying with little Twitter trolls… they're kind of nasty sending out banners and propaganda about me that's disparaging and not true."
She says she has received death threats and the FBI has got involved to track down those responsible.
Her latest claims are part of a string of many she has made about her estranged sister since she was plunged into the global spotlight as the Duchess of Sussex.
Ms Markle has made several appeals to her younger sister trying to get her to reunite with her father Thomas Markle, 74.
The former lighting director missed his daughter's wedding to Prince Harry after undergoing heart surgery.
He has publicly claimed she has since "cut him out" and is devastated he "may never meet'" his grandchild.
Yesterday, the Mail on Sunday revealed the Duchess has made her mark on the royal family.
The divorced former actress reportedly shocked the Queen by wearing such a white dress to her wedding.
She is also up at 5am most days, flooding royal aides about how she can do more in her role.