"Happy Birthday, @RitaOra!" posted the film's official Twitter account. "Rita Ora is Mia Grey, Christian's lively and vivacious little sister. #FiftyShades."
As well as the image of Kosovo-born Rita, stills of Marcia Gay Harden, Andrew Airlie and Eloise Mumford in their roles have also been revealed.
Oscar-winner Marcia and Fantastic Four actor Andrew play Christian and Mia Grey's adoptive parents, Dr. Grace Trevelyan Grey and Carrick Grey, while Eloise, who starred alongside Miley Cyrus in 2012 film So Undercover, stars as Anastasia Steele's best friend Kate Kavanagh.
R.I.P. singer Rita joined the film's cast in December 2013. The British star announced that she would be playing Mia, alongside Jamie Dornan's Christian, while she was attending the British Fashion Awards.
"It's official!," the excited songstress wrote. "I've been cast in @FiftyShades of Grey as Christian's sister Mia."
While Rita is known for her musical releases, she studied acting at the Sylvia Young Theatre School and previously appeared in the 2004 film Spivs and Fast & Furious 6, alongside the late Paul Walker.
A year and a half after it was announced that British director Sam Taylor-Wood would be behind the highly-anticipated screen adaptation of E. L. James' hit book, Fifty Shades of Grey will hit cinemas.
The film, which is the first of the Fifty Shades trilogy, is due for release on Valentine's Day 2015.