Channel Dad's Army or Hitchhikers Guide and Don't Panic!
It's not quite Advent yet (Advent Sunday tomorrow, start of December not until Thursday next week).
You're being normal and she's being premature. Because although you try to plan early, it doesn't work that way because it's not right for you. Starting ever earlier is not the solution, but is a sure-fire way to lengthen the time you feel the stress for.
I'm really sorry to hear about the illnesses in your family, and can see why that might make this Xmas difficult.
Can you find time to sit down this weekend (maybe tomorrow, to mark the start of Advent) and make a list of what you know you want to do to mark the occasion, other things you'd like but depend on health of others, dates relating to schools/workplace/club events, things that other people in your household prize.
That might make it seem a bit more manageable.
(And read all the threads here about households who don't put up their tree until the days before Christmas - Christmas Eve itself for some)