I think I understand what you have described. In your sister's case, her confusion may have been due to high carbon dioxide levels. This may have been caused by the effects of the brain cancer
which can affect the breathing pattern which, in turn, affects the amount of carbon dioxide in the blood. When she was put on a ventilator, her breathing would have been better controlled. This would have lowered her carbon dioxide levels to normal and her confusion would have improved which would have made her more lucid.
If you would like to ask any further question, please do not hesitate to contact me.
Dr K A Pottinger,