Looking to the mouth of a talker early in life predicts expressive communication. We hypothesized that looking at a talker's mouth may signal that infants are ready for increased supported joint engagement and that it subsequently facilitates prelinguistic vocal development and translates to broader gains in expressive communication. We tested this hypothesis in 50 infants aged 6-18 months with the heightened and general population-level likelihood of autism diagnosis (Sibs-autism and Sibs-NA; respectively). We measured infants' gaze to a speaker's face using an eye-tracking task, supported joint engagement during parent--child free play sessions, vocal complexity during a communication sample, and broader expressive communication. Looking at the mouth was indirectly associated with expressive communication via increased higher-order supported joint engagement and vocal complexity. This indirect effect did not vary according to sibling status. This study provides preliminary insights into the mechanisms by which looking at the mouth may influence expressive communication development.