Is locking your mother and sister into a room for a day while you move the crap unceremoniously out into the yard and start demolishing the walls an option? For it seems to me in reading what you've wrote that you are respecting her clutteriness rather than confronting it in a constructive way. If you are saving your father's life, extreme measures may be warranted.
Yeah, I agree with Idude. If you can't lock them in a room, then you'll have to just say to their face that if they can't for a short he'll die, if your mother/sister actually care then they'll at aleast try.