Why is my stepson so disrespectful to me? He had with my niece but my wife thinks he's SO perfect.?

Number one, you can't force your step son to like you. Number two, you married a woman who obviously has no control (doesn't want to control) her child. If you wanted things to be that way you should have married someone else. You and your wife should have set ground rules about raising her son from the beginning, since you didn't you basically shot yourself in the foot. It took both Nick and brittney to get pregnant (just like it took you and your wife BOTH to get pregnant). The so called adults in the house should have had control of the situation, obviously didn't. How you deal with all of it now is your problem I doubt it is going to turn out good though.