"Doria's quiet dignity and grace at the Royal wedding has earned her enormous respect from Meghan's new in-laws", the source tells ET.
Typically, non-royals do not spend the night at the Norfolk estate, and while Middleton's family has attended church with the royals on Christmas morning, they have not spent the night.
The source added: "It's a mark of the Queen's respect for Meghan and an acknowledgment that she doesn't have any other relations in this country - unlike Kate who has the support of a very close family".More news: Apple Isn't Sharing iPhone, iPad, and Mac Sales Figures Anymore
The invitation would would break with royal precedent.
She reportedly said she was "head over heels" with pride as Meghan helped prepare lunch for guests.
And she also told the couple she wanted to spend the holidays with them too. Before extending the unprecedented invitation, the Queen likely considered the fact that Meghan Markle is pregnant for the first time and away from her mother, whereas Kate had access to her mother throughout her pregnancies. Instead, the royals give silly gifts - one year, Middleton reportedly even gave Harry a plastic Grow Your Own Girlfriend kit.