AbstractBackgroundFamily members of people with mental illness (MI) may experience a host of psychological adversities such as increased stress, burden, and reduced wellbeing. However, relatively little is known about siblings. This study aimed to characterise the experience of distress (viz. depressive and anxiety symptoms), burden, and wellbeing in siblings of people with MI.MethodsStudies reporting on quantitative measures of depression, anxiety, burden, or wellbeing in siblings; and/or qualitative findings on siblings’ experience were eligible. The literature search was conducted up until 20th October 2022.ResultsSixty-two studies comprising data from 3744 siblings were included. The pooled mean percentage of depressive symptoms fell in the mild range at 15.71 (k = 28, N = 2187, 95% CI 12.99–18.43) and anxiety symptoms fell in the minimal range at 22.45 (k = 16, N = 1122, 95% CI 17.09–27.80). Moderator analyses indicate that siblings of people with a schizophrenia spectrum disorder experience greater depressive symptoms than siblings of people with other types of MI (β = −16.38, p < 0.001). Qualitative findings suggest that individuals may be particularly vulnerable during their siblings’ illness onset and times of relapse. Limited communication, confusion about MI, and the need to compensate may contribute to siblings’ distress and/or burden. Siblings’ experience of wellbeing and caregiving were closely related.ConclusionThis review highlights the complex psychological experience of siblings and the need for greater research and clinical support for this important yet often overlooked cohort.