The aim of this study was to investigate the development of prosocial behaviors between siblings in a naturalistic setting over a period of two years. This study examined (1) behavioral cues indicating siblings’ needs, (2) responses to siblings’ manifestations of need, and (3) engagement or failure to engage in prosocial behaviors. A total of 40 families with two children were observed in their homes at two time points; when they were 2- and 4-years and then 4- and 6-years of age. Siblings’ interactions were coded for 180 minutes at each time point. Overall, children often failed to express their needs explicitly and consequently, parents intervened frequently to clarify children’s ambiguous manifestations of need, although the children’s helping behavior did not depend on parental intervention. Children’s expressions of need changed over time and varied between siblings. Moreover, some behavioral cues such as nonverbal request and negative emotionality appeared to be more persuasive to siblings, as they elicited prosociality more than other cues. As children got older, they more often addressed implicit cues. Children engaged in helping and sharing more than comforting and protecting, and the theorized association between types of behavioral cues and children’s engagement in different forms of prosocial behavior was found. Children’s engagement in sharing was found to depend on its cost, as well as the ownership status of the contested resource. Overall, the findings illuminate how prosociality develops between siblings. Implications for parents and educators are discussed in terms of possible ways in which this development might be facilitated.