The word "stepsister" is a transparent compound from Old English, combining the prefix stēop- with the noun sweostor (sister). The Old English stēop- carried the specific meaning of "bereaved" or "orphaned," originally describing the relationship created when a widowed parent remarried, often implying the stepchild had lost a parent. Over time, the term broadened to encompass any familial relationship formed through remarriage, regardless of bereavement. Thus, "stepsister" logically denotes a female sibling acquired through the legal union of one's parent with a new spouse who has children from a prior relationship.

Structure: step (related by remarriage) + sister (female sibling) step: From Old English stēop-, meaning "related by remarriage following the death of a parent." It functions as a prefix denoting a relationship resulting from a parent's subsequent marriage. sister: From Old English sweostor, meaning "female sibling." It functions as the core noun denoting the familial relationship.