When Prince William proposed to Duchess of Cambridge, his younger brother Prince Harry was thrilled.
He even famously described Kate as the 'sister he never had' and the pair were often seen giggling or sharing a joke at royal events.
But according to royal expert Duncan Larcombe, Harry and Kate's sibling-like relationship is now "long in the past".
It comes amid a growing feud between William and Harry and the fact that Harry and wife Meghan Markle decided to quit as royals and leave the UK for California.
And now as Kate fast approaches her 40th birthday, Duncan told OK!: "Kate was, as Harry said, the sister he never had and clearly that is not the case anymore.
"There was a time when Harry would have been instrumental in any celebrations or any fun that the couple were having, but those days are long in the past."
Now Duncan added it will be interesting to see if Harry and Meghan make any public message to Kate on her birthday.
He explained: "This is the problem, as the rift continues, as the war between the boys seems to show no sign of finishing, days like Kate's 40th birthday milestone immediately become events that can be contrasted with how it used to be."
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It is unclear exactly what Kate's birthday plans are for Sunday - with many predicting she will have a very low-key celebration due to the Covid-19 situation.
A source told the Mail on Sunday : "There are likely to be low-key celebrations for the Duchess. She didn’t want anything flashy anyway – that’s not exactly her thing – but particularly given the current climate anything is likely to be scaled down."
In August, Kate's sister-in-law Meghan turned 40 and used the milestone to launch her 40x40 scheme in a comedy video with Hollywood actress Melissa McCarthy.
However, according to Duncan, it's not expected that Kate will follow in Meghan's footsteps and announce anything.
He added: "She’s unlikely to because, firstly, it’s just not her way and secondly because of the obvious comparisons with Meghan and this awful continuation of one-upmanship that the palace is so keen to avoid."
