The goal of this study is to propose the Siblings’ Experience Quality Scale (SEQS), a measure for the assessment of cognitive, emotional and behavioral experience of adult individuals having a brother or sister with an intellectual/developmental disability, chronic physical illness or mental illness, on five dimensions: Closeness, Conflict, Jealousy, Self-Marginalization, and Worry. A sample of 213 Italian adult typically developing siblings (ages 18–69 yrs.) was used to demonstrate the psychometric validity of the scale. Results of confirmatory factor analysis, which resulted in high scores of goodness of fit, supported five identifiable factors reflecting the theoretically-based constructs of the SEQS and demonstrated convergence with the general construct of siblings’ quality of experience. Correlations with the Lifespan Sibling Relationships Scale (LSRS), the Beck Depression Inventory II (BDI-II), and the Aggression Questionnaire (AQ) demonstrated the construct, convergent, and discriminant validity. In particular, Jealousy, Conflict, Worry, and Self-Marginalization were negatively correlated with LSRS, while Closeness was positively correlated with LSRS; Worry and Self-Marginalization were positively correlated with BDI-II; and the majority of the SEQS subscales were not correlated with the AQ. Furthermore, siblings of individuals with intellectual/developmental disability showed higher scores on Closeness and Worry, while siblings of individuals with chronical physical illness showed higher scores on Jealousy, and siblings of individuals with mental illness showed higher scores on Conflict. Finally, siblings with higher Self-Marginalization and Worry scores showed higher levels of internalizing problems, as well as anger and hostility. Implications for research and policy are discussed.