They are successors to the British throne, so it seems inevitable that Princes William and Harry's women are constantly pitted against each other in tabloids. The commoner Kate Middleton, now officially Catherine, Duchess of Cambridge, and Cressida Bonas, a socialite with a noble lineage, are reportedly not fond of each other. And it seems that it would take a long time before the two become the best of friends.

Where did the alleged animosity between the two classy ladies start anyway? Apparently, it all started with their sisters.

Before Kate caught the eye of the future king, William and Cressida's older half-sister, Isabella Calthrope, were reportedly an item. Their relationship didn't progress, though, which is fortunate for the now pregnant Kate. Another story claims that Kate and William were already dating in college when Isabella pursued the prince.

And even though Isabella is now married to billionaire Richard Branson's son, Sam, Kate still holds a grudge against Isabella, and therefore Cressida as well.

Cressida, for her part, also doesn't have any love for the Middleton sisters. According to reports, although Harry and Pippa, Kate's younger sister, never hooked up, they have a "flirty" relationship. It was apparently very evident during Kate and William's wedding in 2011, where their siblings had prominent roles as the best man and the maid of honour.

This didn't sit well with Cressida, who only started dating Harry last year, so she allegedly ordered Harry to stop talking to Pippa, which of course had upset Kate.

Kate also doesn't want a "royal chaser" like Cressida for Harry, while Cressida and her half-sisters Isabella and Gabriella think that the Middleton sisters are from the "other side of the tracks."

So basically it's a sisters versus sisters war, and the royal brothers are in the middle.

It's a different scenario when Harry was still dating Zimbabwean national Chelsy Davy, a commoner like Kate who dated Harry on-and-off from 2004 to 2010. Kate and Chelsy had famously gotten on well together back then. Unfortunately for both Kate and Harry, Chelsy didn't want the life of a prince's wife.

Cressida is also said to be having reservations on marrying Harry, even if she has all the requisite credentials. Her mother, Lady Mary Gaye Curzon, was an "It" girl in the '60s and a daughter of an earl, and her father, Jeffrey Bonas, is a businessman.

She also studied at the Leeds University, which coincidentally is also where Chelsy had studied her postgraduate law degree. Unlike Chelsy, though, she is used to mingling in royal circles. She is close friends with Princess Eugenie, Harry's cousin who introduced them together.