In an unprecedented move, Queen Elizabeth II has extended an invitation to Kate Middleton's parents, Carole and Michael Middleton, to be the first-ever non-royals to join her family's annual Christmas gathering at the Sandringham Estate. The bid breaks with decades of tradition, since the celebration until now has been an exclusively royal affair. As the new issue of Us Weekly reveals, Her Royal Highness hopes that the generous overture will encourage Prince William's pregnant wife to partake in the festivities "despite this difficult time." The "difficult time," of course, refers to Middleton's struggles with hyperemesis gravidarum, an acute form of morning sickness. The Duchess, 30, was hospitalized with the condition earlier this month and has since been forced to bow out of several planned engagements as she recovers at home. That said, the royal mom-to-be has been feeling increasingly healthier in recent days. On Dec. 16, she attended the BBC Sports Personality of the Year Awards -- her first official public appearance since announcing her pregnancy -- where she told former soccer player Gary Lineker she was feeling "very well." Two days later, she and her husband, also 30, hosted a low-key holiday lunch for 27 members of their staff at the Bumpkin restaurant in London's Notting Hill neighborhood. Still, insiders tell Us that the Duchess is taking her schedule day by day and won't finalize Christmas plans "until the night before or the morning of. It depends [on] how she feels," an aide says. "And if Catherine doesn't go, William probably won't either."