Pippa Middleton has something special to celebrate this weekend — her 41st birthday!

Kate Middleton’s younger sister turned 41 on Sept. 6, inspiring a look back at the key way she supported her older sibling this summer. On July 14, the Princess of Wales made her second public appearance this year at the Wimbledon Tennis Championships after announcing in March that she is receiving cancer treatment — and Pippa was quite literally right behind her in a show of support.

Princess Kate, 42, her daughter Princess Charlotte, 9, and Pippa had a girls’ day out when they watched the men’s singles final from the front row of the Royal Box, where photographer Karwai Tang told PEOPLE their tight bond was clear.

“You can tell that all three of them were enjoying themselves,” said Tang, who captured several sweet moments between the family, including Princess Charlotte’s look of love for her mother when she was welcomed into the stadium with a standing ovation.

While Princess Kate and Pippa are both known to attend Wimbledon most summers (Kate is patron of the All England Lawn and Tennis Club that’s home to the iconic tournament, while Pippa first checked out the competition this year with her husband James Matthews on July 12), their bond goes much deeper.

Pippa stepped into the spotlight when she served as a bridesmaid at her sister’s wedding to Prince William in 2011 and has been there for her in the years since, joining her at royal events like her children’s christenings and Kate’s Together At Christmas carol concerts.

The Princess of Wales leaned on her loved ones after sharing the news about her health in March. At the time, a family friend told PEOPLE that Princess Kate “doesn’t feel isolated in this because of William, who is right beside her.”

The Princess of Wales’ parents, Carole and Michael Middleton, have been an additional source of help, with Carole accompanying her daughter on school drop-offs. Meanwhile, the princess’ siblings, Pippa and James, also live nearby.

James shared a touching statement on Instagram after Princess Kate announced that she was undergoing cancer treatment, along with a throwback photo of them together as children.

“Over the years, we have climbed many mountains together. As a family, we will climb this one with you too,” he wrote at the time.

Pippa doesn’t maintain a public social media presence — apparently by design. After she and her husband James, an Eton-educated financier, got engaged in 2016, a source told PEOPLE that the couple shared a “deep desire” to disappear from the spotlight in the longer term and looked forward to “having children and leading a quiet life.”

Pippa and James married in May 2017 at St. Mark’s Church in Englefield, England (where Prince George and Princess Charlotte adorably served in the wedding party!). Their family expanded when their children Arthur, 5, Grace, 3, and Rose, 2, were born.

In her professional endeavors, Pippa is the author of the entertaining guide Celebrate and the healthy recipe book Heartfelt, with the latter benefitting the British Heart Foundation, where she serves as an ambassador.