What if it was a 21 year old sibling who has been stabbed but is otherwise is in excellent health?
Forget arguing about technicalities of the law. Just as a thought experiment imagine you are in a situation where you have the final say. Your sibling is a devout JW. They are unconscious. They will definitely die without blood and definitely live with it. It is your decision alone.
What is your call?
I probably would let them have the blood and deal with the consequences.
And cofty, real life is full of "special conditions" that should be considered in making life altering decisions.