The London based musician, who had a small role in Fast & Furious 6, announced the news on Twitter.
"It's official! I've been cast in @FiftyShades of Grey as Christian's sister Mia. #FiftyShades," she Tweeted Monday.
If you were thinking the platinum haired 23-year-old doesn't look much like Jamie, who's playing billionaire control freak Christian Grey, the siblings are actually adopted, according to the book.
Filming for the movie adaptation of E.L James' best-selling erotic novel has already begun in Vancouver. Jamie and Dakota Johnson, who plays Anastasia Steele, were seen filming in a local coffee shop.
Do you think Rita is a good addition to the cast?
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Gallery | Brit Awards 2013 Red Carpet Arrivals