Population-based studies have shown a concordance of breast cancer survival among first-degree relatives (FDRs), suggesting a heritable component. Reasons for such heritability remain to be elucidated. We aimed to determine whether association of breast cancer survival among FDRs is linked to shared patient and tumor characteristics or type of treatment. At the population-based Geneva Breast Cancer Registry, we identified 162 FDR pairs diagnosed with breast cancer. We categorized FDRs into poor, medium and good familial survival risk groups according to breast cancer-specific survival of their proband (mother or sister). We compared patient, tumor and treatment characteristics between categories and calculated standardized mortality ratios (SMRs) and adjusted disease-specific mortality for each group. Breast cancer patients in the poor familial survival risk group were more likely to be diagnosed at later stages than those in the good familial survival risk group. Similarly, they had higher SMRs than those in the medium and good survival risk groups (18.7, 95% confidence interval [CI]: 9.4-33.5 vs. 16.5, 95% CI: 7.5-31.3 and 9.4, 95% CI: 3.4-20.4, respectively). After adjustment for patient and tumor characteristics and type of treatment, women in the poor familial survival risk group were almost five times more likely to die of breast cancer than those in the good familial survival risk group (adjusted hazard ratio 4.8, 95% CI: 1.4-16.4). Our study shows that breast cancer prognosis clusters within families and suggests that the hereditary component is independent of patient and tumor characteristics and type of treatment.