Objective: To examine whether family factors shared by siblings explained the association between education and risk of lung, colorectal and breast cancer. Design: We used conventional cohort and intersibling Cox regression analyses to analyse the association between education and risk of cancer. Setting: Denmark. Participants: We retrieved register data from Statistics Denmark on individuals born in Denmark 1950-1979 with at least one full sibling. The cohorts included between 391 931 and 1 381 369 individuals followed from age 28 for incident lung, colorectal and breast cancer until the end of 2009. Results: In the cohort analysis, low education was associated with an increased risk of colorectal cancer before age 45 and lung cancer, and with a decreased risk of colorectal cancer after age 45 and breast cancer. When compared with the cohort analyses, the intersibling associations were stronger for colorectal cancer after age 45 and weaker for lung cancer. Serious health conditions in childhood/young adulthood did not explain the associations. Conclusions: Family factors shared by siblings confounded some of the association between education and colorectal cancer after age 45 and lung cancer, but not the associations found for colorectal cancer before age 45 or breast cancer.