The aim of this study is to provide the first systematic analysis of differences in the closeness of sibling relations between lesbian, gay, and bisexual (LGB) and heterosexual adults. Background: Across countries, LGB people experience poorer socioeconomic and health outcomes than heterosexual people. Although the reasons for this disadvantage are complex, low levels of family support have been recurrently identified as an important contributor. However, the available literature has concentrated on relations between LGB people and their parents, neglecting other important familial bonds-including sibling bonds. Method: To compare the closeness of sibling relations between individuals with different sexual orientations, this study leverages unique data from an Australian national survey (Household, Income and Labour Dynamics in Australia Survey, n = 13,157 individuals, 35,622 individual-sibling pairs) and multilevel regression models. Results: Key results indicated that-when compared with heterosexual individuals-LGB individuals had less frequent contact with and lived geographically farther from their siblings. The pattern of effects was similar for bisexual and gay or lesbian individuals, and stronger for male