Hi.
Risk factors for the development of breast cancer include age, reproductive history, family and personal history of breast cancer, and exposures to specific carcinogens. You have a high risk of developing breast cancer based on the strong family history. The risk of developing breast cancer by age 70 is about 45% to 87% for those with BRCA1 mutations while it is about 26% to 84% for those with BRCA2 mutations.
Since you are at risk, you may undergo genetic testing. It is very important also to modify lifestyle, exercise, and have a regular consultation with your doctor.
Hope this helps.
I hope you are already starting mammograms because of your family history. And best wishes for your sister.