Dear JoMomma, There is no stage 5 breast cancer. Breast cancers are classified within 4 stages based on a TNM staging system. TNM stands for tumor, nodes and metastasis. If your sister had a stage 4 breast cancer that would mean that there was evidence that cancer had spread to another area of the body, distant from the breast. It may be that she has misunderstood something in terms of a numbering system related to breast cancer?
In regard to your risk of breast cancer, it is increased just based on the fact that your sister has breast cancer. You may want to seek the opinion of a genetic counselor to help you to determine your actual risk and consider a plan for surveillance.
Could she have been referring to the BIRAD scale?