Read Cinderella to the children often. Fear helps keep them in line ...
JOKE, people ... step anythings get the short, slobbery end of the stick in most literary presentations. The only thing you can really do about that is work hard not to live up to the stereotype of the evil-step-fill in the blank ... respecting the kids, respecting yourself, and their bio-parents are probably going to provide a lot better time for all of you. As familial relationships become more complex it's necessary to balance all of these interactions.
(I either said an awful lot or very little in the above.)