I am sorry, but without more specific information about the alleged "abnomormalities in the lungs" it is not possible to even speculate what these abnormalities might be. I agree with your question about how it can be "very serious" without an idea of what it may be. The doctor who described the findings as "very serious" is surely well-intentioned but whatever this is, it can't be any worse than your
fearsFears and phobias of "the worst". The doctor should be able to better describe the likely scenarios, if not to you, certainly to your sister.
Good luck.