Current estimates suggest that by 2015, 60% of college students will be women, a change since 1970 when 59% were men. We investigated family dynamics that might explain the growing gender gap in college attendance, focusing on an ethnically diverse sample of 522 mixed sex sibling dyads from the National Longitudinal Study of Adolescent Health. We examined whether the difference between sisters' and brothers' reports of their mothers' expectations for, and involvement in, their education during adolescence predicted their differential odds of college attendance seven years later. Sisters were more likely than brothers to attend college, and this gap was more pronounced among non-Whites and non-Asians. Sisters also had higher grades in school than their brothers. Although there were no gender differences overall in maternal educational expectations or involvement, brothers reported greater maternal involvement than sisters in non-White and non-Asian families. After controlling for family background factors, the average of siblings' reports of maternal treatment, and differences between siblings' grades, the results revealed that as sisters reported greater maternal educational expectations than their brothers, it became more likely that only the sister rather than only the brother in the family attended college. The difference between brothers' and sisters' reports of their mothers' educational involvement and their odds of attending college showed the same pattern of association but was not statistically significant. These results suggest that within-family social comparisons may play a role in sisters' and brothers' choices about attending college.