I don't see why voicing my concerns is "having issues" no it’s not. This is a bit of a red flag, my DP did this. He could say stuff and discipline my son however if I raised concerns about his kids in any way there would be extreme defensiveness. It is an impossible and very isolated position to be as a SM, and be wary you will be scapegoated.
Reflecting back now, I see that the dysfunctional behaviour in DPs first family got that way because they closed down to ‘outside’ comment.
The fact that they were estranged in the first place is a sign of their own dysfunction.
I’m more inclined to think now, from what you’ve described, that your instincts of feeling very uncomfortable are right. Your DP is not treating you as an equal at all. This may have been manageable before, but now there his other kids are back in the picture, and being manipulative too, it may become unbearable for you.
Your DP by not letting you and him have an equal voice even before this happened, is not going to now listen to you at all. He’s done the opposite, made decisions without you instead. He seems part of the dysfunction, not an innocent player, and therefore will take it out on you if you say the truth. It’s not healthy you are right. But they will all defend their unhealthy behaviour!
I’ve been there. DPs oldest did a shocking thing to both her parents a while ago, estranged them and then did something very worrying. I just voiced that it was worrying when it finally came out. Just that, I did not say a bad word against her. And yet DP and DSD and her mother etc came down on me like a tonne of bricks! They were so defensive that even one word made them all group together and pour all their anger towards me. I felt like I had walked into another country where the ‘worrying thing’ was just normal and everyone treated me as if I had revealed my outsider status by questioning the status quo.
The dysfunction serves a purpose, like your older DSCs. They will have gained power from withdrawing which is now being used. You can see that. Your DP does not want to see that.