Things have changed between Kate Middleton and her sister Pippa after the former joined the royal family in 2011. 

In the 2013 book "Kate: The Future Queen," author royal biographer Katie Nicholl mentioned that the younger Middleton was the competitive one as compared to the Duchess of Cambridge. Despite that, however, they remained close to each other. 

"They had such drive and were very competitive, particularly Pippa, but never against each other," Denise Allford, who coached the girls in St. Andrew's, told Nicholl. "They were very much a team."

Kate and Pippa Middleton's Long-Standing "Secret Competition"

Over the years, it appears that the two brunette beauties had a long-standing "secret competition," as cited by Express UK.

The news came after royal author Andrew Morton claimed that the future Queen consort and the socialite had a deep-rooted sibling rivalry just like other royals. 

To recall, Queen Elizabeth II reportedly had a fair share of conflict with her younger sister Princess Margaret, as well as her son, Prince Charles, against his other siblings. 

Even the modern royal family members have been a target of sibling rivalry--this includes Prince William and Prince Harry's rumored rift. 

In his 2011 book "William & Catherine: Their Story," the two sisters remained secretly "very competitive" with each other but attempted to be subtle about it given that Kate now has more significant roles to fill in the royal family. 

Moreover, Morton mentioned that the royal mom was surprised with the attention that her younger sister gets after her and Prince William's royal wedding. 

"Just as William and Harry are very competitive, Catherine and Pippa are very competitive," the expert stated, adding: "The day was Catherine's day, and Pippa played a significant walk-on part, that was it. I don't think she stole the show. Pippa had an exotic walk-on part."

Meanwhile, to add fuel to the fire, journalist Emma Cowing wrote in "The Scotsman" in 2008 that the mother-of-three feared that her younger sister might overshadow her. 

"Kate was always quite jealous of Pippa. I sensed that she feared being eclipsed by her because Pippa has more natural effervescence, is socially much more at ease and was always popular with everyone, especially James's friends," Cowing mentioned. 

Kate Middleton's Biggest Rivalry?

Aside from Pippa, another highly publicized rivalry of Kate Middleton is none other than Meghan Markle. 

After the former "Suits" star was given the Duchess of Sussex title, rumors sparked that they are not getting along while living under Kensington Palace. 

In the bombshell book "Finding Freedom," written by royal reporters Omid Scobie and Carolyn Durand, the two set the record straight and revealed that both Duchess was never feuding, but instead, they didn't know each other well.

"Meghan would agree with the assessment that the duchesses were not the best of friends. their relationship hadn't progressed much since [Markle] was Harry's girlfriend," as mentioned in the book.

In addition, the royal reporters revealed that Kate did not make an effort to reach out to the former Hollywood actress. 

Moreover, the 39-year-old California native was said to be "disappointed" that Kate, being "an outsider to the Firm," was not able to guide her with the ins and outs of the monarchy. 

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