Cressida Bonas may have won Prince Harry's affections, but his sister-in-law Kate Middleton is not so easily charmed.

The long-running feud between the aristocratic Bonas sisters and the Duchess of Cambridge and her sister Pippa has no end in sight, especially now that the 24-year-old beauty is dating Prince Harry.

Not only is there no love lost between Prince Harry's new lady love and the duchess, but Kate may also disapprove of Bonas' close association with her sister, Prince William's former flame, Isabella Calthorpe-Branson.

And now the rivalry has heated up even more after Cressida allegedly asked her beau to cut ties with Pippa. Although there was never anything more than friendship between Prince Harry and the brunette writer, the flirtatious nature of their interactions did not appeal to Cressida.

"Kate wasn't happy when she heard what Cressida said," a source close to the royals tells Now Magazine. "She told Wills, who had a word with Harry. He told him they were concerned that Cressida's walking all over him."

The royal trio - Prince Harry, Prince William and Catherine - are a tight-knit group and it is understandable for the duchess to be concerned about her brother-in-law's relationship, especially since her own sister is getting involved.

"She was really upset when she heard Cress was trying to control who he talks to," the source added. "She's very protective of her brother-in-law and is worried he'll get hurt."

Despite the doubts from his fellow royals, reports say the red-haired prince is completely enamored by his girlfriend. In earlier reports, royal insiders say Prince Harry has even approached Kate for a bit of relationship advice.

"Harry's so head-over-heels in love with Cressida that, with Prince William's approval, he's enlisted Kate's help to make sure he doesn't sabotage the relationship," an insider reveals to Showbiz Spy.