Usually a sibling, though extramuros relationships are often poorly defined and understood, so the term may be used to describe any family member that shares a significant part of their childhood.
In Anathem, narrator Erasmus discovers his 'sib,' a young woman named Cord. The exact nature of the relationship and consanguinity is never made entirely explicit, but it is apparent from the text that traditions familial obligations among Saecular Arbrans vary widely. Cord shows a great deal of care towards Erasmus, both of whom are apparently of 'baseline' or sline stock, which corresponds to unskilled workers on earth, while Erasmus' friend, Jesry, from a burger (bourgeois, townfolk) family, has much more formal contact and less interaction with his family during the events of the novel.
Throughout the novel, each Ark, or religious organization, seems invested in codifying familial relationships, potentially as the result of the extreme short-term familial relationships exemplified in sline society.