Siblings of children with autism may be at greater risk for psychological problems than siblings of children with another disability or of typically developing (TD) children. However, it is difficult to establish whether autism or the presence of intellectual disability (ID) explains the findings in previous research. Mothers rated the emotional and behavioural adjustment of siblings of children with ID with (N = 25) or without (N = 24) autism. Data were also available 18 months later for siblings of children with autism and ID (N = 15). Siblings of children with autism and ID had more emotional problems compared with siblings of children with ID only and with normative data. Three variables were pertinent: increasing age of the child with autism, having a brother with autism, and being younger than the child with autism. Behavioural and emotional difficulties of siblings of children with autism and ID were relatively stable over 18 months.