My 90 y/o great grandma was just diagnosed with breast cancer and my 17 y/o sister is worried that one of us might get it or another cancer. i'm male.

No Ca: Congenital Br Ca not present if one member develops Ca. If several members involved then genetic testing needed. 90% of breast cancers are environmentally caused such as by MMTV virus . Male members of exposed family rarely get breast Ca but may acquire it if a genetic problem in family. Females acquiring breast Ca have incidence of 1 in 7 although for males much lower.

No reason to worry: Breast cancer does develop in some members of a family but the hereditary breast cancer is rare(affecting less than i 1 in 10 cases of breast cancer in general. Moreover breast Cancer is fairly common Cancer in the older females affecting about 1 in 8 women. So your suister has to think positive and not worry at the present time. Breast cancer is easy to diagnoses and is highly curable(>80% cured)