She announced the news on Twitter on Monday night, while attending the British Fashion Awards 2013.
"It's official!," the excited 23-year-old wrote. "I've been cast in @FiftyShades of Grey as Christian's sister Mia."
Talented Rita studied acting while attending the Sylvia Young Theatre School, and in the past appeared in the 2004 film Spivs, as well as last year's Fast & Furious 6, alongside the late Paul Walker.
The actors were pictured on the first day of shooting, filming a coffee date between 27-year-old millionaire Christian and college senior Ana.
Handsome Northern Irish actor Jamie, 31, was dressed in a dark three-piece suit and an unbuttoned white shirt for the scene, while Dakota, 24, wore a khaki military-style jacket with a blue and white scarf around her neck.
It was announced in June that British director Sam Taylor-Wood would helm the highly-anticipated screen adaptation of E. L. James' hit book.
Production was pushed back by a month after Sons of Anarchy actor Charlie Hunnan dropped out of film, weeks after he was cast as the leading man.
The release date for the first installment of the Fifty Shades trilogy is set for February 2015.