Objectives. To analyze the social competences of siblings of children with ADHD and Autistic Spectrum Disorders (ASD), in relationship with the family system functioning. Method. We studied 20 siblings of children with ADHD, 15 siblings of children with ASD and 20 siblings of normal children in which we applied the following instruments: Child Behaviour Checklist (CBCL)- the competences scales, for the siblings and Family Assessment Device (FAD), for the families, both instruments were completed by the parents. Results. The scores from the competences scales of CBCL are nor significant different in all the three groups of siblings; the scores for the siblings of ASD children are very close to the scores for siblings of normally developed children and a little higher than the siblings of children of ADHD. We found a number or correlations between social competences and family functioning in families of children with ASD.. Conclusions. In family of children with ASD, the functioning of family system is more adequate than in ADHD families. The characteristic symptoms of ADHD can lead to the increase of stress in the whole family, which suggests the necessity of all our attention and the appropriate intervention in the family.