The first two seasons of The Crown are available now on Netflix, and feature a cast that includes Claire Foy, Matt Smith, Vanessa Kirby, Eileen Atkins, Victoria Hamilton, Jeremy Northam, Ben Miles, Greg Wise, Jared Harris, JOhn Lithgow and Matthew Goode.
According to editors at the Evening Standard, the two-time Oscar nominee will join Broadchurch star Olivia Colman, who is replacing Golden Globe-winning actress Claire Foy as Queen Elizabeth II, as her sister in the series which details the monarch's life.
Series creator Peter Morgan has previously said he meant to replace key cast members in later seasons of The Crown to better portray the main characters as they age.
Early production is reportedly under way on Seasons three and four despite that lack of official green light, and should likely expect the show to return next year. Bonham Carter has had plenty of experience playing royals previously. She has not yet signed on to play Princess Margaret, Elizabeth's only sibling and at one point second in line to the throne, but is close to doing so.