An Etymological Dictionary of the German Language/Stief-

by Friedrich Kluge

Stief- in compounds is preserved throughout the Teut. group only as the first component; comp. MidHG. stiefbruoder, -kind, -muoter, -sun, -swester, -tohter, -vater; OHG. stiuf-bruoder, -chint, &c. (Du. stief-broeder, -kind, &c.). Corresponding to AS. steóp-sunu, -fœder, E. step-father, &c.; OIc. stjúpfaðer. That the word was used by itself at an earlier period is indicated by the derivatives OHG. stiufen, irstiufen, bistiufen, ‘to rob one of his relatives (parents or children),’ AS. âstŷpan, ‘to rob.’ All further clue to its early history is unfortunately wanting.