As parents age, well siblings are often asked to assume caregiving responsibilities for their brother or sister with mental illness. However, relatively little is known about how well siblings prioritize sibling caregiving responsibilities with other life demands. We examined well siblings’ attitudes toward self-care and caregiving for their sibling with mental illness (self- and sibling-care) using two cross-sectional samples. The first sample of well siblings (N = 242) was used to examine the psychometric properties of the self- and sibling-care measure (SSCM), designed to assess the degree to which siblings prioritize their own needs and the needs of their sibling with mental illness. A second sample (N = 103) was used to determine the relative contribution of self- and sibling-care attitudes in accounting for variation in well siblings’ reports of personal loss and stress-related personal growth. Results support the psychometric validity of the SSCM and suggest that self- and sibling-care attitudes account for greater variance in scores on perceived personal loss and stress-related growth than demographic or caregiving factors. Our findings support the need to address family care responsibilities and resource limitations through recovery-oriented mental health policies, services, and programs.