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.
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 ....