Despite being a well-documented predictor of children’s cognitive and social development, sibship has received remarkably little attention in the attachment and maternal sensitivity literature. The only study that has examined both sensitivity and attachment in relation to sibship found greater maternal sensitivity but no more secure attachment among first-born infants. In the current study, we sought to examine the same links while testing two related hypotheses: that sibship size relates only to some specific aspects of sensitivity, and that sibship size relates to sensitivity only among certain mothers, namely those who are at risk for suboptimal parenting because of an insecure attachment state of mind. We assessed three dimensions of maternal sensitivity at 12 months and child attachment at 15 and 25 months among 258 mother–infant dyads living in intact biparental families. Compared with mothers who had fewer children, those with more children were observed to be less accessible/available, less positive, but not less cooperative/attuned, when interacting with their infant. These links were moderated by maternal attachment state of mind, such that significant relations were observed only among mothers presenting a more insecure state of mind. Finally, sibship size was unrelated to attachment. These findings suggest that failure to consider different dimensions of sensitivity or important parental moderators may result in the erroneous conclusion that birth order and sibship size are inconsequential for parent–child relationships.