A prefix used before father, mother, brother, sister, son,
   daughter, child, etc., to indicate that the person thus spoken of is
   not a blood relative, but is a relative by the marriage of a parent;
   as, a stepmother to X is the wife of the father of X, married by him
   after the death of the mother of X. See Stepchild, Stepdaughter,
   Stepson, etc.