I am sorry to hear that your family has been so affected by cancer. Fortunately, breast cancer, especially when caught at an early stage, is a very treatable disease with a generally good prognosis. Unfortunately, with any cancer, there is always some risk (even though that risk may be very small) that the disease can come back, no matter how good the initial treatment was. For that reason, your sister's doctor
will want to follow up with her for some time to look for troubling signs.
should have some idea as to what percentage of people will be cancer free based on data collected from similar patients with similar treatments. Obviously, the results of the treatment thus far and any other individual factors will make her situation potentially much different than the predictions that can be made using population based data. That is again why time and careful follow up are so important.
Hopefully, with the experience and data that your oncologist has at his or her disposal you will be able to get some realistic expectation of what lies ahead for your sister in her battle against this disease. Please speak to her doctor for the most accurate prognosis and assessment.