This study tested the hypothesis that, in infant-mother-sibling interactions, infants with older siblings aged 11 to 24 months produce deictic gestures when they are proximal to, or engaging in joint attention with, their mothers more frequently than same-aged infants without siblings. Fifteen infant-mother dyads and 10 infant-mother-sibling triads were individually observed for 15 minutes in a playroom full of toys. Infants involved in infant-mother-sibling interactions produced more deictic gestures when they were proximal to their mothers than infants in infant-mother interactions. Further, infants involved in infant-mother-sibling interactions accompanied their gestures with vocalizations at a higher rate than infants in infant-mother interactions. This result suggests that infants with older siblings monitor their mothers more carefully in interactions in which their sibling is also present, and that they produce deictic gestures in order to effectively elicit joint action with their mothers. Keywords: siblings, deictic gestures, mother-infant-sibling triads, infants, joint attention, physical proximity