An Etymological Dictionary of the German Language/Annotated/Stief-

by Friedrich Kluge

Stief- in compounds is preserved throughout the Teutonic group only as the first component; compare Middle High German stiefbruoder, -kind, -muoter, -sun, -swester, -tohter, -vater; Old High German stiuf-bruoder, -chint, &c. (Dutch stief-broeder, -kind, &c.). Corresponding to Anglo-Saxon steóp-sunu, -fœder, English step-father, &c.; Old Icelandic stjúpfaðer. That the word was used by itself at an earlier period is indicated by the derivatives Old High German stiufen, irstiufen, bistiufen, ‘to rob one of his relatives (parents or children),’ Anglo-Saxon âstŷpan, ‘to rob.’ All further clue to its early history is unfortunately wanting.