Samantha is claiming the book will "expose" her sister.
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.
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 is really like.
Markle 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'."
Mrs Markle is keeping the exact contents of the book a secret.
Last week, Meghan's nephew Tyler Dooley opened up about the trauma Meghan's new role brought to the Markle family in his new MTC series, The Royal World.
He said, "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."
The Royal Family is yet to respond to Samantha Markle's latest claims.
