Sleep problems are common in families raising children with autism spectrum disorder (ASD). Clinicians often depend on parent reports of child sleep but minimal research exists to address the accuracy or biases in these reports. To isolate parent-report accuracy (from differences in sleep behaviors), the sleep of younger siblings were assessed within a 2-group design. The present study compared parent diary reports of infant sibling sleep to videosomnography and actigraphy. In the high-risk group, families had at least 1 child with ASD and a younger sibling (n = 33). The low-risk comparison group had no family history of ASD (n = 42). We confirmed comparable sleep behaviors between the groups and used paired t tests, two 1-sided tests (TOST), and Bland-Altman plots to assess parent-report accuracy. The parameters of sleep onset, nighttime sleep duration, awakenings, morning rise time, and daytime sleep duration were evaluated. Diary and videosomnography estimates were comparable for nighttime sleep duration, morning rise time, and awakenings for both groups. Diary and actigraph estimates were less comparable for both groups. Daytime sleep duration estimates had the largest discrepancy with both groups reporting (on average) 40 additional minutes of sleep when compared to actigraphy estimates. In the present study, families raising children with ASD were just as accurate as other families when reporting infant sleep behaviors. Our findings have direct clinical implications and support the use of parent nighttime sleep reports.