In countries where arranged marriages are common, parents typically want to marry off their oldest daughter first. Because girls in these countries leave school when they marry and thus face limited earnings opportunities when they reach adulthood, the number of sisters one has, and whether they are older or younger, has consequences for a woman's well-being over her life time. Vogl finds that having younger sisters is correlated with a girl leaving school earlier, leading to lower literacy, and to being matched to a husband with less education and a less-skilled occupation. He finds that these cross-sister pressures on marriage age are common throughout the developing world, although their effect on schooling costs varies by country.
The paper by Tom Vogl is titled "Marriage Institutions and Sibling Competition: Evidence from South Asia." The paragraph above is taken from the front page of the NBER (1 October 2012).