Denise Brown said she was "shocked and horrified" to learn that a literary agent for the Goldman family had reached a deal to publish the controversial book.
In a prepared statement, she said she couldn't bear that her sister's two children "will have to be subjected to this step by step manual on how their mother and her friend Ron were murdered."
Brown's condemnation came a day after Los Angeles-based literary agent Sharlene Martin of Martin Literary Management announced a deal to publish "If I Did It."
The publishing house that will print the book was identified as Beaufort Books, a small publisher in New York. Its recent titles include "The Knock at the Door" by Margaret Ajemian Ahnert and "The Presence Process" by Michael Brown.
"We will be working diligently to not only publish this book well, but to honor the memory of the victims of this terrible crime: Ron Goldman and Nicole Brown Simpson," Beaufort president Eric Kampmann wrote in a statement announcing the deal.
The book will be published with Simpson's original manuscript intact and include commentary, said Michael Wright, a spokesman for Martin. The Goldmans, the publisher and Martin will contribute portions of sales proceeds to the newly formed Ron Goldman Foundation for Justice to help victims of violent crime, he said.
The new plan to publish "If I Did It" came months after HarperCollins dropped the book because of public outrage.
Last month, a federal bankruptcy judge awarded rights to the book to Goldman's family to help satisfy a $38 million wrongful death judgment against Simpson.
In her statement, Brown accused Goldman's father, Fred Goldman, and other family members of hypocrisy for publishing a book that he had called "disgusting and despicable" when Simpson first planned to publish it.
David Cook, an attorney for Fred Goldman, called Brown inconsistent for denouncing the publishing plan after lawyers for her and her family sought a share of possible profits from the book for themselves.
Simpson has maintained his innocence in the killings of Nicole Brown Simpson and Goldman in 1994. The former football great, who now lives near Miami, was acquitted of murder in 1995.