It says family will join her. IMO it makes sense. Sandringham is always so crowded anyway whereas Windsor means Andrew and family and Edward and family can stay at their own homes while Anne and family and Charles and family can take up more rooms. Windsor has bigger rooms including state apartments that would allow for better distancing and ventilation. Also more staff live in or around Windsor than would need to trek across to Norfolk for Sandringham. Logistically it just makes sense to me.
Absolutely, logistically it solves many issues and for HM's comfort, it's ideal. I could see this becoming a semi-permanent arrangement going forward. I know I wouldn't mind seeing the family show up for Christmas at St Geroge's along the lines of how they do for Easter.