I'm not sure anyone can tell you what you need to know. Stage 4 is certainly treatable these days and advances are being made all the time. The best place for an answer would be a talk with her Oncologist; providing that she will allow such a discussion. An answer, if there is one, could only come from her Physicians who know every detail of her particular case. Regards ....
Dear MarySmith: Unfortunately, it is impossible to predict how long a person will live. Metastatic breast cancer is generally considered incurable but not necessarily untreatable. There are many possible treatments and depending on a variety of factors, age, general health, and response to treatment, women can live weeks, months or even years. Her oncologist may have a better feel for the situation since he knows all the particulars of her case. You may ask your sister for permission to discuss this with him/her. However, even the best guess can be wrong. You may also see if there is a social worker that works with your sister’s oncologist. This could be an excellent resource for helping both of you through this emotionally as well as logistically.