Depending on the severity of their disabilities, children with Down syndrome (DS) and with cerebral palsy (CP) may remain pre-symbolic for prolonged periods of time. When interacting with pre-symbolic children, communication partners have a role in identifying which of their behaviours are communicative, to be able to respond to those behaviours and maintain reciprocal interaction. To date, most research on these children's communication development has been conducted within the context of mother-child interaction. Seldom have they been observed interacting with other family members, and in interactions other than dyadic, despite these interactions also occurring daily.To explore and compare the interaction of mothers and siblings with pre-symbolic children with DS and with CP in dyadic and triadic contexts.Twelve pre-symbolic children with DS (aged 1;10-5;04 years) and 12 with CP (aged 1;09-5;07 years), and their mothers and siblings participated in this study. They were recruited from early intervention centres from the West Coast of Peninsular Malaysia. Children were observed as they engaged in three play interactions: mother-child, sibling-child and mother-sibling-child (triadic).Children produced the most pre-symbolic communicative behaviours during mother-child, followed by triadic and lastly sibling-child interaction, suggesting that the mother's presence encouraged children to communicate. Mothers created a more facilitative communication environment for the children than did siblings, by directing high rates of initiations towards them and by taking turns that paved the way for the child's next turn. Although siblings' low rates of interaction did not provide children with as many opportunities to produce communicative behaviours, it encouraged them to produce high proportions of initiations. During triadic interaction, mothers and siblings had to direct interaction towards two communication partners, causing them to direct less interaction towards the children with DS or CP. There was no significant difference in mother and sibling responsiveness to children's communicative behaviours across interactions and disability types, suggesting that mothers and siblings might have adapted to the children's various early communicative behaviours.From this study, it was evident that mothers and siblings of pre-symbolic children with DS and with CP engaged them in reciprocal interaction. Findings on how mothers and siblings influence pre-symbolic children's communication suggest the need to involve them in assessment and intervention for these children.