Why is the press trying to course rivalry and bad feeling between Kate & Pippa Middleton?

There is one way guaranteed to cause irritation between two sisters and that is to compare one unfavourably with another. That is what the press are doing with the Countess of Cambridge and her sister. So why do the press want to cause a major fall out within their families? A relationship between sisters no matter how loving can be difficult enough. Never mind a press g-t shoving their gutter noses in