I am a step dad to two 6 year old twin boys, They seldom do what I ask them to do. Help?

Recently I was putting them to bed while there mother was out. They started to play up and generally ignored me, I found my self asking them repeatedly, I tried distraction, the soft approach, and negotiation, I wont bribe or get angry, I am an ertive parent but I fear that one day they are going to say you cant make me your not my dad. What can I do to make them see me as a figure of authority, befor its too late?