Owens has held her seat since 2008. She is up for re-election in 2020.
Grant will handle small civil cases, growing her career. She was appointed by Alabama Gov. Robert Bentley on Friday.
Owens is just 4 minutes older than Grant. Both are married, and each has a 6-year-old daughter. Their husbands are childhood best friends.
Owens and Grant were raised by single mom Loretta Bitten, a librarian for the Birmingham Library.
Jennifer Ardis, communications director for Bentley, said in a statement, "Ms. Grant is highly qualified, motivated and prepared to be a district judge. The governor's office found out about her twin sister during the interview process. Public service seems to be a trait that runs in her family."