It was a girls' night out for sisters Kate Middleton and Pippa Middleton in London Wednesday as the high-profile pair caught up with each other at a charity shopping event.

Fellow shoppers at the Burlington Arcade Charity Christmas Bazaar were caught off guard when the Duchess of Cambridge and her younger sister showed up to support the event and quickly posted photos of the pair on Twitter to prove their proximity to the royal and royal-in-law.

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"Kate and Pippa Middleton spotted at @BurlingtonArcad @CountryTown #BurlingtonBazaar," wrote @ClaireGongMuse.

"Feeling festive with Courvoisier cocktails @BurlingtonArcad in good company of Kate Middleton & Lupo," posted @CourvoisierUK.

Lupo would be the black cocker spaniel puppy Kate Middleton reportedly received as a 30th birthday gift earlier this year from her husband, Prince William, and it was he who proved to be the real surprise guest of the evening, and the only exception to the apparent "girls' only" rule.

The duchess, 30, arrived to the event dressed for the cold weather in dark jeans, a brown jacket and black suede boots and with Lupo and her royal protection officer in tow. Pippa Middleton, 29, arrived a bit later, on her own, also in jeans but paired with a blue leather jacket, printed scarf and black knee-high boots.

The roughly $30 admission fee to the event benefited the Ormiston Children and Families Trust, a charity for disadvantaged youth in the East of England.

The Wednesday night sighting marked the first time the two sisters have been seen at a public event together since June, according to British media reports. Both sisters attended the wedding of a close friend of Pippa's near their childhood hometown in late September.

Kate Middleton also attended that event stag, without William, who was, as he was presumed to be Wednesday night also, fulfilling his duties as a search-and-rescue pilot with the Royal Air Force.

Pippa Middleton has been busy since then making appearances and headlines of her own with the publication of her debut book, the party planning guide "Celebrate."