The simple reason Kate Middleton has not visited Pippa Middleton and her new baby yet.

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Pippa Middleton and her husband James Matthews welcomed their first child into the world on Monday, but new aunt Kate Middleton has not yet met her nephew.

A spokeswoman for the couple said their baby boy was born on Monday at 1.58pm local time (11.58pm AEST). Both he and their mother are reportedly healthy.

The boy’s name or photo has not yet been shared.

A spokesman for the Duke and Duchess of Cambridge said in a statement they were “thrilled” for Pippa and her husband.

Kate has yet to visit her sister and nephew in hospital – but don’t worry, she has a good reason. The Duchess has been very busy at work.

Kensington Palace confirmed Kate took part in an unannounced engagement for one of her charity initiatives Early Years, on Tuesday, the day after Pippa gave birth and the same day her and Pippa’s brother James Middleton visited the hospital.

The meeting was hosted at Kate and Prince William’s Kensington Palace home and was private, with no photographs taken. Although it was not announced until after the event had finished, it had probably been planned for months.

Possibly even before Pippa announced she was expecting.

No other details have been released, but it was previously reported that Kate was working on a solo charity campaign for disadvantaged children.

The Daily Mail reported: “She is bringing together experts from academia, education, health and other fields to work on how to help families tackle anti-social behaviour, addiction and mental health.”

Kate’s been busy with official engagements since she officially returned from maternity leave earlier this month, following the birth of her and Prince William’s third child Prince Louis.

She’s a busy woman, but we’re sure she’ll make time to meet her first nephew as soon as she can.