Hi - Femara keeps cancer cells from bonding with the estrogen they need to grow. Eventually, since they're unable to grow, they die. However, like any cancer drug, there's no guarantee it's 100% effective. Since it's a relatively new drug, there isn't any long-term information about effectiveness or side effects; right now, they're recommending women remain on it for at least 7 years, and possibly indefinitely, if it seems to continue to work. Good luck to both of you—I'm glad your sister has been given a good prognosis, that's very encouraging! - PJH