The Saturdays singer took to Twitter to show her excitement and posted a photo of herself in an elegant platinum twobirds dress, accompanied by her other sister Laura.
"Blonde Power!! #KingSisters #Bridesmaids," wrote Mollie alongside the snap, which shows the siblings wearing matching chignon hairstyles and holding white rose bouquets. "@MollieTheSats best sisters ever :-) gorgeous bridesmaids xxxxx," replied her sister Ellen.
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As the adaptable twobirds bridesmaid dresses can be styled in 15 different ways, Mollie wore hers as a one-knot gown tied across the shoulder. Laura, meanwhile, opted for the classic halter neck style, reminiscent of when Lady Gaga acted as a bridesmaid for her friend's wedding in June and wore the pink blush halter gown.
Throughout the day Mollie couldn't conceal her excitement for her older sister, who got married to long-term boyfriend Harry Thurman on a sunny day in Surrey.
"Omg she’s MARRIED!!!!!! Party over hurrrrrrr. #KingThurman, " tweeted Mollie, soon followed by another post, "My beautiful sister on her big day," alongside a photo of the bride and groom looking loved-up on the dance floor.
Mollie, 26, has been dating American music producer Jordan Omley since February this year. The couple first met while working on The Saturdays' album together, and have been going strong since.
Jordan has met Mollie's family and flown over to the UK a few times, and recently paid the chart-topping singer a surprise visit for her birthday in June.
Mollie has previously said she wouldn't settle for anything less than a true gentleman and has always wished for a fairytale romance.
"I love an old-school gentleman," she said. "Picking me up for dates, sending flowers, holding the door open, I love it all. I think a man should treat you like a princess."
The young starlet certainly played the part at her sister's wedding in the gorgeous twobirds gown which she had bought from Miss Bush Bridalwear in Ripley.
The £250 dresses, popular with celebrities, make up three different collections — the Classic, the Rosette and the Hi-Low. The Classic is available in three lengths and 20 catwalk-inspired colours, perfect for any occasion, while the Rosette adds a little sparkle to the dresses with chiffon rosettes hand-sewn on to the gowns.
The Hi-Low is new to the range this year, and features a stylish, strapless dress available in 20 colours with the possibility of wearing a sash.
All dresses are made and stocked in the UK.