Details emerge on Meghan and Kate's 'extraordinarily tense' relationship

Meghan Markle and Kate Middleton's reported "tension" is reaching a boiling point one year after the now-sisters-in-law famously spent Christmas together with their husbands, Prince Harry and Prince William.

"They really loved Christmas and all had a wonderful time," a source told People. "I think the two women got on -- they are definitely not best friends, but it was a really special time for them all to spend together."

The two couples spending Christmas with the rest of the British royal family came just two months after Harry and Meghan announced their engagement to the world. Since then, Meghan has asserted herself behind-the-scenes, reportedly becoming "difficult and demanding," sending emails to staff at 5 a.m., urging her husband to step out from his brother's shadow and even causing Kate to leave a "stressful" dress fitting for Princess Charlotte ahead of the royal wedding in tears.

WATCH: Kate Middleton gushes about Meghan's baby amid reports of 'tension' between them

Adding to the speculation? The fact that Meghan's assistant, Melissa Touabti, suddenly quit six months after the wedding.

Insiders close to the Windsors insist that part of the "extraordinarily tense" relationship between Meghan and Kate stems from their drastically different backgrounds.

"They try their best to get along, but they are two people from wildly different places, so it is perhaps inevitable that they will rub each other the wrong way," a source told The Sun.

As much as "Kate toes the royal line," Meghan has been "thirsty to meet the public and show just what an asset she can be" to the royals, Christopher Wilson wrote for the outlet.

Of course, it's not just Kate and Meghan who have had their fair share of tense moments recently. Speculation about bad blood between the duchesses comes amid news that Harry and Meghan will move out of Kensington Palace and into Frogmore Cottage at Windsor Castle, 20 minutes outside of London. The decision to move out of Kensington Palace -- where Will and Kate live with their three children -- even though they had the option to take over the 20-room Apartment 1 from the Queen's cousins, the Gloucesters, raised more than a few eyebrows.

SEE ALSO: Why Harry and Meghan are moving out of Kensington Palace

Not only that, but it also pointed to a growing separation between William and Harry. The brothers, who are famously close and grew up in the spotlight together without their mother Princess Diana, have ever-diverging roles in the royal family, with William inching closer to the throne and Harry continually pushed back in the line of succession.

"They have had quite a few arguments," an insider told The Sun. "This is about Harry pulling away from his brother."

As they continue to prepare to become parents for the first time, perhaps Harry and Meghan's upcoming move will provide some much-needed space between the two.