Cognitive and adaptive behaviour abilities early in life provide important clinical prognostic information. We examined stability of such skills in children at high familial risk for ASD who either met diagnostic criteria for ASD at age 7 years (HR-ASD, n = 15) or did not (HR-non-ASD, n = 24) and low-risk control children (LR, n = 37), prospectively studied from infancy. For both HR groups, cognitive skills were consistently lower across time than those of LR children. HR-ASD children showed increasing difficulties in adaptive behaviour over time compared to LR children, while the HR-non-ASD children showed no such difficulties. This pattern of change may inform our understanding of developmental profiles of HR siblings beyond core ASD symptoms.