I don't think Charles will have his siblings and their children with him for Christmas at Sandringham at all.
I do think he will have Camilla's children and grandchildren there instead.
Of course William will be at Anmer and Harry will either stay with his brother or father.
The 'walk' will simply be Charles and Camilla, William, Kate and kids, Harry, wife and kids and no one else. Camilla's kids may or may not go to the church but they want walk there with the rest of the royals.
Of course Charles may not even have Christmas at Sandringham as he currently spends less than 24 hours there over Christmas. He may decide to have Christmas and New Year at Birkhall where he spends New Year now.
Christmas at Sandringham, "the walk", or even summer at Balmoral may be family traditions, but they don't compare IMHO to Trooping of the Colour, the Christmas broadcast, or a constitutional ceremony like the State Opening of Parliament, all of which Charles is bound to keep. I see family traditions changing under Charles and, maybe even more so, under William and Catherine, but more institutional events that make sense to keep will probably be kept.