While recent research has begun to address the effects of family support on transgender and/or nonbinary youth (TNY), almost no studies have directly examined how cisgender siblings in families with TNY navigate their sibling's gender disclosure and affirmation within both their families and their larger communities. We conducted an exploratory secondary analysis of in-person, semi-structured interviews with 15 adolescent and young adult siblings (age 13-24 years) of TNY from the northeastern United States from the baseline wave of the community-based, longitudinal, mixed methods Trans Teen and Family Narratives Project. Interview transcripts were analyzed using immersion/crystallization and template organizing approaches. Analyses yielded three main themes: gender-related beliefs and knowledge, peri- and post-disclosure family dynamics, and assessing responses to their sibling. Subthemes included anticipation of their sibling's TN identity, expectations post-disclosure, participants' level of involvement in gender-related family processes, perceptions of changes in family relationships, concern for their sibling (including a high degree of attunement to gender-affirming name and pronoun usage), and concern for themselves. Findings from this study suggest the need to engage directly with siblings of TNY to further elucidate their intrapersonal, intra-familial, and extra-familial experiences related to having a TN sibling and determine their unique support needs. Implications for families, clinicians, and communities are discussed.