This research concerns the siblings of individuals with severe disabilities. This research described the sibling's relationship with his or her mother (denoted here as the mother-child relationship) and the sibling's relationship with the family member with disabilities (denoted here as the sibling relationship), and this research sought to study the features of these two relationships. Semi-structured interviews were conducted with 18 siblings of individuals with disabilities. Interviewees described their mother-child relationship and their sibling relationship from early childhood to the present. Based on these accounts, five types of mother-child relationships were identified― “actively involved,” “good,” “improving,” “uninvolved/withdrawing,” and “entangled.” Similarly, three types of sibling relationships were identified―” good,” “improving,” and “uninvolved/withdrawing.” The two relationships were categorized as positive, negative but improving, or negative. Even if a sibling had a positive mother-child relationship and a positive sibling relationship, triangulation can occur and the standard sibling order can be reversed. In families of an individual with disabilities, these problems remain as the family system functions. Facilitating de-triangulation is essential so that a sibling can form stable relationships with his or her mother and a family member with disabilities.