I was in those situations when i first moved in. My wife would not back me up when SS and I had disagreements. She just told him that I am just different than them. I am more of a calm person where they are very argumentative. SS yelled at his Mom and she would just back off. I would try to implement some simple rules to reduce the drama but mom would get mad at me or not reinforce them.
With all that said, it has gotten better with SS and myself. We are at this point communicating compared to the first few years where we spoke maybe 3-4 times per month. My issue now is that he is 16 and whines when he is asked to do something. Something as simple as putting the food away and it makes me crazy. I also hired him to do some work for me and he asked for and advance. I told him no... that he gets paid when the work is complete. I am trying to teach him some good business lessons. I wrote out the rules before we started so he couldn't complain about me not giving him money.
When I said no he flipped out. So I fired him. Of course his Mom got defensive, as she always does, and told me that was too harsh. She threw out all these reasons why he flipped out on me and gave me his teenage reasons. That frustrated me more.
I feel that since he is my stepson it makes me more upset. As well as the fact that his mom rarely backs me up and she treats SS like a little Prince. I think she is afraid to hurt his feelings. My opinion is that she is only damaging him by letting him get away with this stuff.
One of the problems is that when he complains to her she gives in. I am very frustrated and don't even want to be around this kid. Maybe if he was my kid I would feel different but since he is a step child I have less of an emotional attachment.