the.com/stepbrother

means The son of your stepparent, brought into your family through a parent's remarriage rather than by blood.

from A straightforward Old English compound: "step-" comes from a family of words (steop-) that originally carried the sense of bereavement or loss — a stepchild was once an orphaned or bereaved one, brought into a new household. "Brother" is the deep, ancient kinship word, a cousin of Latin frater and Sanskrit bhratar, stretching back to Proto-Indo-European. So the whole word literally layers "a brother gained through loss and remarriage" onto one of the oldest family terms we have.