Aunt Meghan and Uncle Harry will be on hand to dote on the royal baby when the prince or princess arrives.
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The couple are neighbours of the Duke and Duchess of Cambridge at Kensington Palace, living in Nottingham Cottage in the grounds of the royal residence.
US star Meghan Markle recently hinted at having her own children with Prince Harry.
On a trip to Belfast, the American former actress joked when she was shown an innovative range for newborns: "I'm sure at some point we'll need the whole (lot)."
In an interview in 2016, Markle said becoming a mother was on her "bucket list".
"I can't wait to start a family, but in due time," she said.
Harry has also said he would love to have children.
The topic cropped up in the couple's engagement interview.
"You know, I think one step at a time, and hopefully we'll start a family in the near future," the prince said.
Markle has been spending time getting to know William and Kate's eldest children, Prince George and Princess Charlotte.
The celebrity, who shot to fame playing paralegal Rachel Zane in the American drama Suits, already has two "fairy god-daughters".
She described the children of her close friend Benita Litt as such in a 2016 post on her since-deleted Instagram account.
The bride-to-be and Prince Harry, who will wed in Windsor on May 19, will be among close family expected to attend the christening of William and Kate's baby in a few months time.
Australian Associated Press