Functional aspects of infant-directed speech (IDS) in face-to-face interaction play a central role in infant development (Venuti et al. 2012b) and are important in shaping infant responsiveness (Goldstein et al. 2003), yet have rarely been examined and there is little consensus on specific infant outcomes. Infant siblings of children with autism are at increased risk of developing this disorder characterised by socio-communicative impairments and present particular challenges for parents in interaction. Using a prospective video analysis design, this paper had two objectives: to investigate maternal functional speech styles in interaction with these infant siblings, and to analyse the relationship between maternal functional style and infants’ cognitive, language and socio-communicative development. The study found significantly lower rates of maternal responsive-contingent utterances to the infant siblings and significant associations between maternal IDS and infant test scores only for the typical dyads.