Background: The study examined relations between a number of variables regarding typically-developing adult siblings of individuals with intellectual and developmental disability: involvement in the lives of their siblings with disability, personal resources (self-efficacy and sense of coherence), loneliness, and adjustment. Method: Participants included 99 siblings of individuals with intellectual and developmental disabilities who completed questionnaires examining involvement, personal resources (self-efficacy and sense of coherence), loneliness, and adjustment. Results: Results indicated that siblings who are more involved and perceive their efficacy and coherence as higher and loneliness as lower, experience higher levels of mental wellbeing and lower levels of mental distress. Personal resources also meaningfully predicted siblings' adjustment, and mediated the relations between involvement and wellbeing. Conclusions: The current study can serve as a basis for professional and current knowledge for the construction of intervention programs to strengthen siblings' personal resources: efficacy and coherence as resilience factors that promote adjustment.