SamIam,
Being that you are all "on the wagon" and shit I think it is entirely ok for you to bow out gracefully saying you truly need to manage your very real holiday stress by excusing yourself from Bill's gathering. Be gracious and thankful, but be firm in taking care of your needs, vis-a-vis sobriety and avoiding triggers that would make you, and just about everyone else, want to seriously bevv-up.
Bill should understand. Especially if his last name begins with a W.
BTW. You don't owe it to anyone to take care of them unless they are your own underage spoor.