Meaning of STEP- in English

STEP-

() 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.

Webster's English dictionary.      Английский словарь Webster.