Cheri, the treatment plan for your sister depends on whether she had two different kinds of cancer:  breast and brain, or one type: breast cancer that has spread to her brain.  If she has breast cancer that spread to her brain, that would be considered Stage 4 breast cancer.  Often mastectomy is not part of the treatment for Stage 4 breast cancers because it is more important to treat the place where the cancer has spread.  

These days people often live a long time with Stage 4 cancer because there are so many new treatments to keep them going.  An honest talk with her oncologist about what she can expect from her treatments would be a good idea.  Balancing quality of life issues versus extending life is an important consideration.  No one can tell you what to expect for sure.  Sometimes people live a lot longer than their doctors expect with a good quality of life.