Princess Catherine’s only brother James Middleton has finally broken his silence months after his world-famous sister’s shock cancer diagnosis.

Princess Catherine’s younger brother James Middleton has shared a rare insight into his private life while announcing a new book about a beloved family dog.

Middleton, 37, is the only brother of Kate and Pippa and is married to French financial analyst Alizée Thevenet.

On Friday, Kate’s brother penned a heart-breaking tribute to his late dog Ella, who is also the subject of an upcoming book.

I wish moments like this could last forever,” Mr Middleton wrote via Instagram on Friday.

Alizée, Inigo and all the dogs came to hear me speak for the first time about my book Meet Ella.

“Ella introduced me to Alizée, a few years later we got married, another year or so later Inigo arrived.

“Ella and Inigo never met but every moment Ella is with us. So if it wasn’t for that moment…I would not have been able to capture this moment.”

James is understood to be close with his world-famous sister, with the entire Middleton family rallying around Kate, 42, following the royal’s cancer diagnosis.

Sources previously said the couple have been receiving help and support from the Princess’ parents, Carole and Michael Middleton, as well as her siblings, Pippa and James,” society magazine Tatler revealed last month.

The comments by Kate’s brother come after a Lieutenant from the Household Division weighed in on the Princess of Wales’ absence from the upcoming Trooping the Colour celebrations.

Kate will skip the military parade on June 15, including her usual role as Inspecting Officer and honorary Colonel of the Irish Guards.

Lieutenant Colonel James Shaw, Brigade Major of the Household Division, who designed this year’s Trooping spectacle, said he was “so proud” to have Catherine as colonel and wished the royal well amid her cancer treatment.