Samantha Markle, 57, is suing her half-sister in federal court.
She argues the Duchess of Sussex caused her emotional stress by claiming she was raised as an only child.
Meghan's claim came in the infamous interview with Oprah Winfrey she took part in, alongside husband Prince Harry.
Samantha's case asserts that Meghan made her look bad during the high-profile sit-down with Oprah, widely recognised as one of the most influential interviewers in the world.
Samantha Markle and Meghan Markle
GB News/PA
Meghan Markle with husband Prince Harry
John Stillwell
She takes issue with Meghan's claim that she was raised without any siblings, arguing that it serves as a "humiliation".
The royal's lawyers say Meghan's meaning was that she bore a "feeling" of having been raised alone and did not state it as a fact.
Samantha, they argue, is playing on her previous relationship with the royal for personal gain.
Now, Judge Charlene Honeywell has set a date of October 2023 for the trial, barring a settlement.
She explained: “The purpose of this order is to discourage wasteful pretrial activities, and to secure the just, speedy, and inexpensive determination of this action."
Judge Honeywell also issued a schedule for filing deadlines and estimated a trial length of five days in her Tampa courtroom.