Hello Beccyand welcome.  I am so sorry about this devastating news for you and your family.    It is difficult to make time estimates with any certainty because even with cancer patients who have the same cancer at a similar stage can vary enormously in their prognosis.  I hasten to add that I have no medical training but am only speaking from (a) my own experiences of caring for cancer patients and (b) the similar stories set out on this forum.  However I would suggest that you go by your gut feeling to some extent; if your sister is not receiving any treatment and appears to be getting weaker then perhaps her children should know.  I am guessing that she is of an age where her children are grown-up (but of course I could be wrong in which case it is even more important that someone explains to young children what is happening).  A medic could give you an estimate but even that would not necessarily be definite.    If someone we love has a terminal illness I think most of us would want to know as soon as possible (but they don't have to tell her that they know if this is really important to her).  Sorry if this is not a very helpful response.  Best wishes to you and your family.  Annie