The Sister Study is the only long-term study of women aged 35 to 74 whose sister had breast cancer. It is a national study to learn about environmental and genetic causes of breast cancer. In the next 3 years, 50,000 women who live in the US and who have had at least one sister with breast cancer and do not have breast cancer themselves will be asked to join the study.
You can learn more about it at their website.
I'll be encouraging both my sisters to sign up.