Objective Anorexia nervosa (AN) is often first diagnosed in the adolescent years. The treatment with the greatest evidence during this time is family‐based treatment (FBT). In FBT, siblings are expected to attend treatment sessions; however, sibling well‐being during this time has not been well researched. This study aimed to explore sibling well‐being when the ill child was initially diagnosed with AN and after FBT had been completed. Method Eighty‐five parents and 55 siblings of adolescents with AN completed the Strengths and Difficulties Questionnaire at diagnosis. In addition, 88 parents and 46 siblings completed the Strengths and Difficulties Questionnaire after finishing treatment. Results Mothers and fathers reported siblings to have lower levels of conduct problems in comparison with population norms. Mothers also reported lower levels of prosocial behaviours. Siblings reported higher levels of emotional difficulties and hyperactivity in comparison with their peers. There were no differences in reported psychosocial well‐being of siblings between diagnosis and following FBT. Conclusions Siblings of adolescents with AN have poorer psychosocial adjustment than their peers, both before and after FBT. Clinicians and parents are encouraged to be aware of sibling difficulties and seek additional support if required. Copyright © 2016 John Wiley & Sons, Ltd and Eating Disorders Association.