Sharing is an important indicator of internalised prosocial values. We examined predictors of sharing of 302 preschoolers with their younger siblings in a one-year longitudinal study. Sharing was observed during different home visits, once with father and once with mother. We examined the following predictors: both children's externalising behaviour, observed parental sensitivity, and situational factors. Preschoolers' sharing was stable and increased with age. Preschoolers shared more when sharing was preceded by a structured interaction with a parent compared to free play with an unfamiliar adult. At age 4 they shared more in fathers' presence than in mothers' presence. Neither parental sensitivity nor child behaviour were related to sharing. These findings demonstrate stability and the importance of situational factors in the development of prosocial behaviour.