Hi Sandrelle,
I too am a step parent of a 12 year old, almost 13 daughter. Like you her mom isn't to much of problem. I tend to be the more involved maternal figure so to speak. My step daughter has been part of life since she was 2. I do not have any of my own children and have always raised and treated her as if she were my own child. I do find myself frustrated like I don't do enough or could and should be doing more.
We are lucky that she is a good kid, good grades, rarely talks back, and has a great attitude. But the few times that she has happened to test the waters it was nipped in the butt rather quickly. For me most of the testing happened early on in our relationship and her dad squashed it. As a family unit (me, her dad and her mom) we raised her to understand that we are her parents and she will respect and listen to all 3 of us.
Just let him know what the consquences of his actions are, and let his father know that these are the consquences also. For example if he talks back he gets his phone taken away for so many days. Give him a warning the first time he does it so he knows you don't like the behavior if he continues or does it again implement the consquence. Stick to your guns, be firm and don't back down.
Best of luck, I think 12 is a hard age. Still a kid, but almost a teenager. Around here 12 is the seventh grade, so it's jr high, changing classrooms, more homework, more choices for activities. Meeting new people at school. Hopefully its just an adjustment period and he will grow out of it quickly.
I just want to add my step daughter lives between both our house and her mothers. Dad is the custidiol parent w. joint custody. We both live in the same school district so she spends equal time between the two.
Jenn