step-

CIDE DICTIONARY

step-,  [AS. steóp-; akin to OFries. stiap-, stiep-, D. & G. stief-, OHG. stiuf-, Icel. stjp-, Sw. styf-, and to AS. āstēpan, āsteópan, to deprive, bereave, as children of their parents, OHG. stiufen.].

 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.  [1913 Webster]

OXFORD DICTIONARY

step-, comb. form denoting a relationship like the one specified but resulting from a parent's remarriage.

Etymology

OE steop- orphan-