Birth order supposedly influences individuals' cooperative attitudes: firstborns are more family-oriented and favor their kin, while laterborns are more likely to turn to non-kin. However little direct experimental evidence exists concerning costly resource sharing between full siblings. The present study investigates sharing decisions with respect to a monetary resource by full sisters when either: (i) interacting with an unknown individual or (ii) their sister. A total of 112 sisters from 56 different families participated in an economic experiment regarding the distribution of an actual monetary reward. In line with kin selection theory, the results showed that participants favor their sister over strangers. Additionally, firstborns invested more costly resources in their sister (than laterborns) while they knew that the latter did not.