Purpose <p>This study aimed to identify profiles of autistic youths’ sibling relations and examined if social-ecological variables (i.e., youth characteristics, family and caregiver functioning, peer relations, academic performance) were associated with these profiles.</p> Method <p>Caregivers (<i>N</i> = 2,142; 88.1% mothers) of autistic youths aged 6–17 years (<i>M</i> = 11.07 years; <i>SD</i> = 3.17; 80.1% male) completed electronic measures assessing social-ecological variables and youths’ sibling relations. We used a latent profile analysis (LPA) to define sibling relation profiles based on the qualities (emotional support, companionship, conflict, and criticism) of relations between autistic youths and their closest-in-age siblings. We performed ANOVAs to compare sibling profiles on social-ecological variables.</p> Results <p>The LPA yielded a 3-profile solution: a positive group (18.2%), a negative group (17.2%), and a low engagement group (64.5%). ANOVAs and χ² analyses revealed no between-group differences on age, gender, history of co-occurring cognitive impairment, or autism characteristics. Youths with positive sibling relations had more adaptive family and caregiver relations than did youths with low engagement sibling relations, who, in turn, had more adaptive family and caregiver relations than the youths with negative sibling relations. Youths with positive sibling relations had more friends and closeness with friends than did youths with low engagement or negative sibling relations.</p> Conclusion <p>Most sibling relations among autistic youth fit a low engagement profile based on caregiver report. Positive sibling relations were linked with positive functioning in other social-ecological domains. The nature of these linkages warrants further investigation, particularly using longitudinal, multi-informant, and mixed-method designs.</p>