You need to leave the idea alone and move on. Just because you have the feeling, it doesn't mean that you should or have to act on it. One reason is that your stepdad and mom have a committment to one another through marriage. Secondly, he is effectively in a father's position and is not available to you in that way. Third, you don't have to be with someone all the time. Take the time to move on and you will be glad for it.