The medical term is Pulmonary Oedema. There can be many causes for this. You could use Dr. Google but would need to know her medical history to ascertain the specific cause and outcome for your sister
I agree with bednobs, we are dealing with e pleural effusion NOT water on the lungs ( pulmonary oedema) and the common cause 50 years ago would have been TB but today is either pneumonia or cancer.