This link can tell you more about the way breast cancer is staged.  Breast feeding is considered one factor that reduces the risk of breast cancer.  The causes of breast cancer are still being studied and there are more theories than definitive answers.

It's hard to say how much your risk for breast cancer has been changed by your sister's diagnosis.  Generally speaking, premenopausal breast cancer is more likely to impact your risk.  The number of other people in your family with breast cancer is also a factor.  If your sister is post-menopausal and if there are no other cases of breast cancer in your family, then your risk and your nieces' risk is slightly increased.  You will want to mention your sister's breast cancer to your own doctor and discuss the specifics for your family.

I'm an eleven-year Stage III breast cancer survivor, so I know how frightening this diagnosis can be.  There are many treatment options available to your sister and every reason to hope that she will get well.