I'd agree that some family counceling is in order. The adults (mother and step-father) need to work as a team and be in agreement, otherwise the kids will play them off against each other and win every time. Never let the kids divide and conquer.
While I'm not a parent, I do understand that you have to be careful with what you say, and you have to follow through with it every time. Otherwise the kids will learn that you don't mean what you say, and that you will waffle if they put you on the spot and make you live up to your word. Depending on the history 2 weeks sounds a little bit much, but if that's what he said, then that's what he has to do. I'm a fan of making the kid stew a little bit while I calm down and rationally think through what I'm going to do. Oh, and they are grounded (no friends, phone, etc...) while I'm thinking.