Those who say things dont make sense have not learned the way of the reader:
1. Only the author is right
2. Only the story is right
Not to forget the most important;
3. Just because you think something would happen or it would make sense for it to happen doesn't mean it will.
Okay, NOW I understand her intentions it was to make sure Qiu Chuxue survived the initial fears the people would have towards her fever, plus destroying her sisters ability base(?) before parting ways. Oops almost forgot the third reason reuniting the best buds again.
„Although it was dark, she could see things clearly“... ? Suddenly she can see when the day before she couldnt distinguish between zombies and humans the day before when it was „already dark“ and „close to midday“??
I really did not like the cold poisoning of her sister. I would prefer a stronger antagonist than a weaker one, but maybe author is planning for this to backfire.