In matching monochrome outfits, the royals had coordinated for the occasion in Monte Carlo – the Monaco Fight AIDS Summer Gala.
Mother-of-three Stephanie showcased her slim legs in a black miniskirt which she teamed with a loose-fitting white blouse, flat black sandals and a white clutch bag.
Her daughter Pauline, 19, took a leaf from her mother's style book wearing high-waisted black trousers and a flowing white top. She gave the sophisticated look a splash of colour with towering red wedge sandals.
Stephanie's only son Louis, 20, proudly escorted his mother and sister on the red carpet, looking stylish in a casual suit. The seventh-in-line-to-the-throne famously shies away from the limelight but he looked carefree as he posed for photographers.
Their sun kissed glows showed that the trio have clearly been making the most of the summer on the Mediterranean coastline.
Stephanie, the youngest child of the late Prince Rainier III and Grace Kelly, is president of the charity.
The latest ball is the third of its kind in the last few weeks in the tiny principality.