Meaning of STEP- in English
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 Revised Unabridged English Dictionary. Пересмотренный полный словарь английского языка Webster. 2012