I think more than petty, jealous and impatiant you are being unrealistic. First let's think about that, why would it be petty to expect a man to parent his children and the one he chooses to live with as well as expecting all children to respect his fiance/wife? Why would you be jealous, he does not treat your son differently than his own children, he equally ignores what the children need which is structure and discipline? Impatient, you have waited from day one for him to step up without a huge confrontaton? This man has established these behaviors with his children for fifteen years. You tend to blame mom because of the way the kids behave and because you love dad, but he is just as responsible for his lazy and rude children. "They have no rules, chores or discipline at their Mom's. In our home, we all have to contribute positively." And yet later you note that they do nothing, you wait on them like hotel staff and they keep you up at night because they determine when they go to bed and whether they bathe.
I have been a stepmom for 19 years. I would have told the kid to get the hell out of my spot not waited for dad to man up because frankly after 15 years of being a parent, that obviously is not going to happen.
He is not going to change, he is not going to parent his children or yours, you are it. If you are comfortable being a single parent when you are married, then stay with him. If you do not think you can handle it no matter how much you love him you need to walk away. It is what it is.
If you decide to stay, you need to suck it up. If the kid is up till all hours with the Xbox take it away. If he takes it back, destroy it. Dad just might get involved if he sees it as a monetary thing, or maybe he will be the type of dad that will just buy him a new one and blame you for breaking the one he had rather than making the kid responsible for his choices or to require him to earn the money to replace it.
My stepson was not around very much until we got custody of him because of geography. He was not a problem until he lived with us full time. His father also traveled. When he was home he pretty much relegated me to the role of one of the kids, he didn't want to hear about it instead of recognizing that the child needed to be parented. You cannot be afraid that the children will not like you. My stepson and I argued nearly every day, normal teen stuff, put things in the laundry, don't put a dish with food on it in the dishwater, average stuff. He only threw it in my face that I was not his mom once and he is still close to me and doesn't speak to his father. I actually kicked him out about 12 years ago and that was the one time his dad backed me up, said it was up to me whether he was allowed back home. If they know you are afraid that they won't like you, etc., you are toast and will never gain any control over them. Believe it or not, many parents that have raised their kids right face the same thing during the teen years so it is not just the step-parenting and lack of discipline, it is just a hard age.
I made the mistake of believing that all of the discord with the X was because she was a bad mother, had cheated on him, etc. but you know, he was not innocent in the problems in the relaionship. Nor the dysfuncional way they interacted, and still do more than 20 years later, they cannot have a phone conversation that does not devolve into an argument. You need to give things a great deal of thought. The two year mediation was strictly her fault or are there things he won't give on either? Those boys are around quite a bit, if things never get any better than they are today, is this how you want to live for the next twelve years?
Is this also fair to your son? Even if he can intellectually grasp why you expect certain behavior of him but not of his step-siblings and also allow them to be disrespectful to you, how is he to manage all the feelings that generates? Possibly resents you, resents and/or dislikes his step-brothers, definitely resents step-dad for failing to parent and not protecting you. Might be a little full blown anger mixed in there as well, but because he is a good kid, he will sit on it because he loves you and wants you to remain with a man who obviously makes you happy.
What happens if one of the boys gets physical with you? Your son would certainly try to intervene and would you be prepared to leave or stay in a home within which now you are also physically frightened? My stepson was a very sweet young man at 11 but by the time he was 17 he was a very angry young man who threw me on the front lawn and pinned me down by my neck while my then 5 and 2-year-olds stood at the front door watching.
I tend to be a person who hopes for the best but quite often what we get is far from that and you have to consider whether you can live with the worst scenario before you get married or even remain committed living together. Do you want your son's disposition to change? He remains okay now, but continung to live in this environment will do what? Your son will be gone to college in three years, do you want home to be a haven or a place he flees?