the.com/stepsibling

means The child of your stepparent — someone you're related to through a parent's remarriage rather than by blood.

from A transparent compound of "step-" and "sibling." The "step-" prefix is the surprising part: it comes from Old English "steop-," rooted in words for bereavement and loss (related to "astieped," meaning orphaned or deprived). Originally "step-" marked a relationship created by a death — a stepchild was a bereaved child taken in, a stepmother the woman who stepped into a dead mother's place. "Sibling" itself is also Old English ("sibling," from "sib," meaning kinship or relatedness), but it nearly vanished, surviving mostly in dialect until scientists revived it in the early 20th century as a tidy term for "brother or sister." So the modern word fuses a very old grief-prefix with a freshly resurrected kinship-word.