The aim of this study is to understand siblinghood in old age through the perspective of Czech older adults. Forty-one in-depth interviews were analyzed, and four themes characterizing siblinghood among older persons were identified in the data: (1) Siblinghood as a unique bond, understood through genetic relatedness, the sense of belongingness, shared memories, and awareness of being the last members of the family of origin. (2) Being consanguineous and, as a result, irrevocably bonded by mutual commitments. (3) Being emotionally supportive and ready for help if needed and as resources allow. (4) The rules of doing siblinghood: the genealogical rules vs. the rule of family of procreation priority; the rule of genetic relatedness vs. the selectivity rule, or rather the preferences of particular siblings; the rules of “good siblinghood” (being interested in themselves, mutual respect, no envy).