Purpose: The present study explored the extent to which early prelinguistic communication skills predict expressive language in toddlers with autistic siblings (Sibs-autism), who are known to be at high likelihood for autism and language disorder, and a comparison group of toddlers with non-autistic older siblings (Sibs-NA). Method: Participants were 51 toddlers (29 Sibs-autism, 22 Sibs-NA) aged 12-18 months at the first time point in the study (Time 1). Toddlers were seen again 9 months later (Time 2). Three prelinguistic communication skills (i.e., intentional communication, vocalization complexity, and responding to joint attention) were measured at Time 1 via the Communication and Symbolic Behavior Scales Developmental Profile-Behavior Sample. An expressive language aggregate was calculated for each participant at Time 2. A series of correlation and multiple regression models was run to evaluate associations of interest between prelinguistic communication skills as measured at Time 1 and expressive language as measured at Time 2. Results: Vocalization complexity and intentional communication displayed significant zero-order correlations with expressive language across sibling groups. Vocal complexity and responding to joint attention did not have significant added value in predicting later expressive language, after covarying for intentional communication across groups. However, sibling group moderated the association between vocalization complexity and later expressive language, such that vocal complexity displayed incremental validity for predicting later expressive language, covarying for intentional communication, only within Sibs-NA. Conclusions: Results indicate that prelinguistic communication skills, in particular intentional communication, show promise for predicting later expressive language in siblings of autistic children. These findings provide additional empirical support for the notion that early preemptive interventions targeting prelinguistic communication skills, especially intentional communication, may have the potential to scaffold language acquisition and support more optimal language outcomes in this population at high likelihood for a future diagnosis of both autism and language disorder.