Why Kate Middleton might skip Princess Eugenie's wedding.

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Princess Eugenie’s wedding is the next big occasion on the British royal family’s calendar on October 12. But an equally momentous event on the other side of the Duchess of Cambridge’s family may cause her to miss her cousin-in-law’s big day.

You see, Kate’s sister Pippa Middleton, who announced her pregnancy earlier this year, is reportedly due to give birth in October.

And while we don’t know Pippa’s exact due date, if the birth of her and husband James Matthews’ first child does fall on October 12, Kate would choose to be by her sister’s side, The Express reported.

(As an aside, it's also very unlikely Pippa and James would be attending the wedding of Princess Eugenie and Jack Brooksbank.)

Kate isn't the only member of the British royal family in doubt for Princess Eugenie’s wedding.

According to a “well-placed” insider, who spoke to People magazine, Prince Philip is tossing up whether to head back to St George’s Chapel for the second royal wedding of the year.

If the 97-year-old doesn’t attend, Princess Eugenie will have her parents to blame, according to the source.

“He made a supreme effort for Harry. I would be very surprised if he did go, because he is so estranged from Fergie,” they said.

“I think he will decide at the 99th hour.”

Pippa Middleton wore one of our favourite outfits at Meghan Markle and Prince Harry's wedding in May.