Sisters and brothers play an important role in the lives of their siblings with intellectual and developmental disabilities (IDD), starting in childhood and commonly increasing in adulthood. The current study compared typically-developing adult sisters' and brothers' level of involvement in the care of their siblings with IDD. Beyond this, the study examined differences in sisters' and brothers' readiness for primary caregiving - including their readiness for guardianship and belief in their sibling's independent decision-making, loneliness, and self-efficacy, and how these variables predict their involvement. Participants included 175 Israeli adult (aged 18 years and older) siblings - 118 sisters and 57 brothers of individuals with IDD (Intellectual Disability and/or Autism Spectrum Disorder). The participants completed online questionnaires. Results revealed sex differences, with sisters reporting more social involvement, greater belief in their sibling's ability to make independent decisions, lower levels of loneliness, and higher self-efficacy than brothers. Additionally, hierarchical regression analyses revealed a unique sex difference in predicting siblings' involvement, with readiness for the future role of guardianship positively predicting involvement only among brothers. Variability in sisters' and brothers' involvement needs to be considered when exploring transition planning to primary caregiving, including guardianship and supported decision-making.