Pippa Middleton's ass caught the world's attention at the Royal Wedding of Kate Middleton to Prince William, on the 29th of April, 2011, where she was chief bridesmaid
That third picture, where it's "Pippa" with her back to the camera, it's actually Kate. Anyone who looks at her face can tell. Also, in other pictures, Pippa's bikini bottom is larger. If you look at the last picture, her's has ties on the side. Kate's doesn't.
Actually, Pippa's sunglasses are all black, whereas Kate's have white on the sides, as shown in the picture in question.