Few published studies have reported on the psychological well-being of family members of individuals with Rett syndrome (RTT). Eighty-seven mothers of girls and women with RTT completed a questionnaire survey about their daughters’ behavioral phenotype, current health, and behavior problems, and their own and a sibling’s well-being. Mothers reported increased anxiety but similar levels of depression when compared to a normative sample. Across all problem domains on the Strengths and Difficulties Questionnaire, child and adolescent siblings (n = 39) were reported by mothers to have fewer difficulties than a normative sample. The severity of their daughters’ RTT behavioral phenotype predicted increased anxiety and stress for mothers. Increased RTT daughters’ current health problems predicted more maternal perceptions of positive gain.