James Middleton reveals his son Inigo using hand-me-downs from Prince George and Prince Louis

"They are milestones, because [my sisters] remember when their child was wearing something, and how old they were - it takes them back," Middleton told Hello! magazine.

"It's been a lovely thing for them," he added.

The familial clothes-sharing not only provides Inigo with a fresh set of outfits - but it also helps to foster a touching connection between the royal relatives.

Middleton expressed gratitude for Inigo's connection to his older cousins and lauded their positive influence on his son.

He said: "He's fortunate to have older cousins who can now pick him up and show him things, and as he grows up, they'll be able to look out for him."

The 37-year-old author also praised his sisters' parenting skills, stating, "It's been wonderful seeing them become mothers and being so dedicated and strong.

"I'm in awe of that," he added.

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Middleton acknowledged learning from his sisters' experiences, and joked: "I've been fortunate to see how my sisters have done it, and take tips along the way – some I take on board, and others I'm like: 'Hmm'."

James is set to release his new book next week - and it chronicles his relationship with his cocker spaniel, Ella, who, he writes, played a crucial role in saving his life.

Middleton's idea for the book emerged after his wife Alizee discovered she was pregnant with Inigo at the same time as Ella died aged 15.

He wanted to share his memories of Ella with his son, creating a lasting tribute to the dog who had been such an important part of his life.

The book covers various adventures, including trips to Scottish mountainsides, royal weddings, and even how the spaniel played a part in Middleton meeting his wife.

Beyond its role as a memoir, Middleton's book also serves as a heartfelt story of survival.

The Princess of Wales's brother candidly discusses his struggle with clinical depression and suicidal thoughts, revealing how Ella played a crucial role in his recovery.

By sharing his experiences, Middleton aims to raise awareness about mental health issues and the therapeutic benefits of canine companionship.

And in another show of support for this cause, the 37-year-old is backing Dogs Trust's inaugural "Walk for Our Old Friends" event in October.

The hike, a 3.5km "walk of love", will see dog owners come together to traverse a patch of woodland in a commemorative trek to "remember our old friends", the charity says.