The Duchess of Cambridge made the blue dress famous in the spotlight way before in 2010 when she was engaged to Prince William. Now, her sister made another replica of the same style, but of different color that also captures the eyes of many.
There has been somewhat like a voting in the web of who's who, who made an elegant look. As for me, I personally like the blue color because it suits to the skin tone of Kate Middleton and for Pippa, the dress is just okay for her.
Sisters are really like sisters. But I think they don't compete in any ways or for many ways. I guess they are just apt to what they think are best to wear. But they really look pretty amazing!