Unfortunately therapy is not an option in my partner's mind. They tried some therapy with him some years ago. They visited several therapists and none of it helped. Many of them accused my partner of causing the behavior. So she's pretty sour on the topic. I've tried to bring it up saying that I'd take care of the details and get him to/from sessions. No luck.
One thought about that is if SS12's behavior is intolerable to you, then it's OK for you alone to seek family therapy to learn new ways to cope. Nobody else has to cooperate, go along, or even think it's a good idea. You get to decide your limits for dealing with SS12 without further support, so if you are reaching a breaking point or "something has to change" point, it's OK for you to consult with a family therapist on your own. There's no requirement to bring along the rest of the family when you're getting help for a family issue from a family T. It'd be nice -- but it's not like "well, she doesn't think it will work, so I guess I can't go".
We've consulted with the kids' former T on our own, without the kids (as Mom would likely have undermined our efforts to bring the kids at that point). There was no pushback from the T of "well, if you're having an issue with the kids, then until you can make them come, I can't meet with you".
The "why" is definitely a rabbit hole. I do my best to avoid that subject as I can and focus on things that we could do together to help. One thing she's identified is that he acts out when he's got an audience. So one thing I proposed was to instruct him to do his chores then we leave the house for a bit while he gets it done. Then he has no one to bounce off of. She seemed to like that idea.
Has she been consistent about liking that idea and trying it, and being OK with you trying it? That would be great to have a united front not only about "hitting is a problem" but also about "and here is one thing we agree we will do". Baby steps
I really think I need to figure out a way of stepping in when I see things running off the rails with the two of them. They both ramp each other up very quickly sometimes.
If I were having that question, I'd definitely be raising it with a family T. It's a complicated situation being in a stepparent role -- you aren't the parent, yet you live in a household where you're in a parent-type role, and additionally, even if "technically" it's the bio parent's job to manage discipline, you're in this situation where you're getting hit with the shrapnel from the blowups.
When your partner is calm and regulated, have you approached her about if she wants you to intervene or assist when she and SS12 go at it?
