I absolutely agree, no one, absolutely no one, would turn down the Queen´s invitation to a Sandringham Christmas.....my guess is the same as yours Iowabelle, she just wasn´t invited......
Going to Balmoral for a visit is quite a different story and perhaps the Queen is curious to actually talk to the girl, it has been quite a few years after all.
Actually it might not be a personal invitation, Prince William has a little place of his own on the grounds and has asked permission to have a friend or friends to stay.
No doubt we will hear all about it soon in the tabloids.