While the family has been described as a gender and sexuality factory, most research on gender socialization in families focuses on how parents contribute to or challenge cisheteronormativity in their childrearing and acceptance, rejection, or ambivalence toward their LGBTQ children. This leaves the potentially supportive role of nonparental relatives, such as siblings who act as important peer and family resources and influences, understudied. Drawing on 73 qualitative interviews from a mixed-methods longitudinal study with LGBTQ youth in South Texas and the Inland Empire of California, I analyze the meaning-making narratives that LGBTQ youth use to evaluate their siblings as supportive family members. I find that siblings engage in a range of practices that affirm or resist family cisheteronormativity, with consequences for LGBTQ youth’s wellbeing, sense of safety, and housing stability. This study advances research on gender as an accomplishment through family interactions by demonstrating how siblings hold LGBTQ youth accountable to cisheteronormativity as well as use accountability processes to challenge other family members to recognize gender and sexual diversity.