J.M.
T., I am a stepmother, too. My son is now 35 and doing well.
But when my son turned 11, I felt like a different person entered his body---I know this sounds crazy--but I have no other words to describe it.
I'll will pass on some advice given me at that time. It was so straightforward and logical that I was doubtful that it would work. The counseler told me to give all disciplanary duties back to my husband. Basically--give your stepson back to his father. But only disciplanary duties---you should only have to talk about fun stuff with your son.
It worked--my husband was a little surprised about the amont of time needed to raise his son. And it all worked out well in the long run.
To keep from butting in when they were discussing difficult things, I would hold my lips together or leave the room. I know it sounds silly-but it works.
Good luck, J.