So, first things first; I am a step-dad. I have known the boys (ages 6 and 8 - lets call them Tim and Robert, respectively) for about 18 months now, and they do look up to me as a father figure (calling me dad, listening and following my advice - e.g. "Listen and think", etc.) and I have been told by my partner (their mother) that she sees the way they look up to me.
However, I am not exactly certain of their history, beyond the knowledge that at least one previous partner (not sure if it was their actual father or not) was very abusive. Again, exactly how, I'm not certain, but from what I have been told, I believe it was primarily physical - I have been told that their mother was often placing herself between him and the boys.
That said, I don't know what, or even if that has an effect on the boys, primarily Tim. He has told me about their "bad dad", but not much more than a name and maybe one or two enjoyable activities (like playing roly-polys on the trampoline). What I do know is that he is a victim of bullying, but again I'm not sure the extent - he may simply have a bully at school, or he may have a dysfunctional friendship circle of "bully" friends that occasionally pick on each other.
I am working on understanding all of this better, but that will take time; especially to do with the previous partner, as that will require revisiting some very distressing memories on behalf of their mother (we have spoken about therapy, and she is slowly making efforts to do so).
What I do know for certain however, is that Tim is bullying Robert. Tim is very much being the "alpha" in their relationship. Tim leads, and Robert must follow. If anything goes wrong, or they are caught out, Tim immediately tries to blame it on Robert. They both have a fear of being disciplined, especially physically (probably due to their history), so I make the effort to talk to them, help them understand the rules, help them think about what they should be doing, like telling the truth, not taking things that don't belong to them, being fair, listening and being nice to each other, etc.
I know that the boys listen to me; I come home some days, and they are being rascals, not listening to their mother, procrastinating on their chores, doing what they want and disregarding the rules. So I "put my foot down" by firmly reminding them of what they have been asked to do, point out their misbehavior, and stay on "overwatch" to make sure they do as they have been asked. Depending on how upset I get, I try and come back afterwards to talk to them calmly about what upset me and their mother, to again try to help them understand what they did wrong. From this, I have seen a change in their behavior around me.
What I am concerned about is the unintended side effect of the "adult male role model" they have had to deal with before, and the absence of one in the year or so between the old partner and myself, and in particular the "instinctual behavior" of Tim. It resurfaces around new people; whenever my parents take care of them, he reverts back to taking things that don't belong to him, he tests the boundaries of "who is in charge" by doing what he wants, not listening, and any time that "blame" is placed, it's automatically Robert's fault, so that Tim doesn't get the consequences.
Tim is very smart, and at the moment, he uses his intelligence to do the wrong thing - sneak around, find where things are and take them, and even to figure out locks and passcodes to get to what he wants. So I personally have a mild doubt whether his "good behavior" around me is just for show, or if he simply associates the "good behavior" lessons to me specifically, and only remembers them when he thinks of me/I am around.
So what can I do to make the lessons stick better with Tim (beyond just sticking at it), and how can I teach him about his behavior towards Robert is unacceptable?