Article continues below advertisementPrince Harry and Meghan Markle are heading into what insiders tell OK! will be a "nightmare Christmas," as tensions with both the royal family and Meghan's estranged relatives cast a shadow over the seventh consecutive year they are excluded from the Sandringham festivities.The Duke and Duchess of Sussex, based in Montecito, Calif., are believed to have received no invitations to join King Charles and senior royals for the annual family gathering in Harry's homeland of Britain, despite brief optimism earlier this year that relations could thaw.