Princess Beatrice and Sarah Ferguson join Princess Eugenie for important cause this evening

Princess Eugenie founded The Anti-Slavery Collective in 2017 along with her good friend Julia de Boinville.

The duo also host a podcast titled Floodlight where they discuss ways to combat modern slavery.

Princess Eugenie, her older sister and mother all looked unified in black dresses this evening.

Eugenie, 33, wore her Peter Pilotto Brooch-Embellished Gown in Black Satin.

The princess has worn this dress twice before, both times for different weddings.

The dress likely has special meaning for Eugenie, as the designer Peter Pilotto also designed her wedding gown back in 2018.

WATCH NOW: Princess Eugenie has an Anti-Slavery podcast

Princess Beatrice, 35, stepped out this evening to support her younger sister wearing a Self Portrait Tailored Crepe and Chiffon Midi Dress.

She has also worn this dress twice before, both times in 2022.

Another royal who has worn this outfit was the Princess of Wales, who donned the dress for the Royal British Legion Festival of Remembrance last year.

Its description says: "This midi wrap dress is cut from tailored crepe, chiffon and fine lace.

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"Designed with a longsleeve bodice that is fitted at the waist and falls to and pleated midi skirt.

"This style is lined with a self-fabric belt and pockets."

The Duchess of York also looked lovely this evening, re-wearing a black shoulder-less dress that she wore in Sweden last year.