I grew up in a blended family from age 11 up. (My father passed away when I was a young child.) From a kid's perspective, 2 things stick out in my mind about what helped my step-dad make this work. In no particular order, 1) parents (the married adults in the house) should discuss all issues and discuss them in private, however, the bio parent of the child at issue makes the final decision. IOW, talk about all things amongst yourselves, without involving or informing the kids of details, but if it's your bio-child, you have final say, if it's his, he has final say. 2) Have projects around that involve every member of the household and make every member (ok- motivate them) to help with them. When my mom and step-father first got married and I learned he'd have at least one of his children move in with us, it was frightening and stressful. Plus, my mom moved us to a new house in the midst of it all. It's very unstabling, no matter how happy the adultsd might be. Finding an adult to be in a kid's life doesn't replace a missing parent- it never can- there's a lot more to it than that. But like my step-father did, if you can let all kids know that noone is trying to replace someone who can't be there but instead, the adults want to offer to help but won't force it. OK- I guess I inserted that one as a third but I do think it's important to let all kids involved know that and to reinforce that. The second one was really meant to be another thing my step-father did that I thought was a great idea.....and still do.
The adults ( my mom and he) bought an old house in order to fit us all in. It needed work done on it- it's the only way they could afford it. He told us kids that if we pitched in and helped with the work, it would save the money from hiring someone else and we could afford to go on a vacation the following summer. Now that might sound like just some way to save money and get us to work. It was much more than that- it made each of us feel like we had a part in that house, it belonged to each of us equally, after we invested "work" in it, and we all wanted to take care of it because we had an investment in it. Plus, we got the vacation- at first motivated by our individual desires but it unified us as a family.
I'm not suggesting you go buy a house and move if you don't need to; I'm suggesting you find a way to unify everyone and help everyone feel like they have an equal stake in it and that they can work together to get something individually and as a "new family". No one is singled out as "the bad one" or "the good one"- it's a unified family.
Just passing on what I think worked when my step-father "joined" my family.....