The Duke of Sussex will also be celebrating his first Father's Day this weekend.
Baby Archie is bringing the royal family together.
"It's absolutely the case that the brothers did fall out," Nicholl says. "I was told by a very senior royal source that actually they weren't talking to each other at one point. I certainly think now that Archie is here, it has probably really helped to improve the relationship. William is over-the-moon that his brother is a dad."
There had been reports that Harry and Will were not exactly on the best of terms. Last December, a royal source told ET that "tension does still exist" between the brothers after the Duke of Cambridge originally voiced his "concerns" about Meghan when she first started dating Harry. According to our source, the conversation didn't go over well with Harry, and they still had "not mended completely." Then at various events earlier this year, there was obvious distance between them.
Harry and Meghan Markle welcomed Baby Archie in early May. The two have been settling into parenthood quite nicely, enjoying their time with their baby boy. While Meghan is on maternity leave, she did step out over the weekend to attend Trooping the Colour with the rest of the royal family.
As far as Meghan and sister-in-law Kate Middleton's relationship? "I hear that Kate has made herself available to Meghan if she needs advice, if she needs any help, and Kate said to her, I'm here. I'm here for you," Nicholl also tells ET about the Duchess' being there for each other.
Meanwhile this weekend, Harry will be celebrating his first Father's Day. According to Nicholl, it will most likely be a "low key" day for the new parents.
"They celebrated Meghan's first Mother's Day quietly in the grounds of Frogmore," she explains. "She released that beautiful picture of Archie's little feet. It wouldn't surprise me at all if they celebrated with another picture on Instagram."
"This is going to be a really important day for Prince Harry. He waited for a very long time to become a father," she adds. "There were moments when he feared he would never become a dad. So to mark Father's Day with baby Archie is going to be really special, and if we're lucky, who knows, we may even get a glimpse of the new prince on Instagram."
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