The bond with a sibling is potentially the longest lasting of all personal relationships. Siblings play a significant part in each other's lives over the entire life course. They are drawn together because they share a genetic, family, social class and historic background, but they are also independent as they follow their own career paths and start their own families. This research focuses on how solidarity in the adult sibling relationship can be explained by examining which factors influence the exchange of instrumental and emotional support. A general exchange perspective is taken in which siblings are expected to take into account the costs and benefits of exchanging with each other. Will siblings provide more help when they are more similar, or rather when they are different from each other? To what extent is support exchange in sibling relationships comparable to support exchange in friendships? Will people help out a sibling no matter what, or are there moral conditions to this support? And in what way does the relationship with parents influence support exchange among siblings in adulthood? These questions are addressed using information on family relationships obtained from a large representative sample of the Dutch population, providing new insights into the conditions under which adult siblings in the Netherlands support each other.