This study focused on the quality of the relationship between women with a lifetime anorexia nervosa (AN) diagnosis and their sisters, in the context of family functioning. Participants were 112 sister pairs including a woman with a lifetime diagnosis of AN, and 356 sister pairs with no history of an eating disorder. Participants completed the Sibling Relationship Questionnaire, Family Environment Scale and Eating Attitudes Test‐26. We hypothesised that sister relationships would be less favourably assessed by women with an AN diagnosis than by their sisters, and less favourably assessed overall in the AN sister pairs than in the healthy sib‐pair controls. These hypotheses were confirmed. Also, the AN sisters viewed the family as less cohesive and more conflicted than their non‐affected sisters. For the AN sisters, there was an inverse relationship between level of current pathology and how favourably they viewed the sister relationship. Sister relationships may suffer during AN and improve as the pathology recedes. This would mean that the quality of sister relationships has clinical significance and may serve as an indicator of recovery. Copyright © 2016 John Wiley & Sons, Ltd and Eating Disorders Association.