I do have sympathy for the new mums/mums-to-be, because these aren't unusual feelings, and they are obviously so strong at a time when one feels pretty vulnerable anyway. The thing is, they aren't terribly rational feelings. But they aren't 'wrong', and you aren't selfish for having them. You do have to figure out how to deal with them.
I have begun convinced, after seeing so many of these threads and the information that comes up in subsequent posts, that maybe these feelings are a sort of displacement of other concerns. Something is making these new mums feel insecure about their role, and their children's role, in the family. Now, in a few cases, maybe it is just the case that the new mum is a terribly insecure person. In most cases, though, there may be other things that are feeding that insecurity - not feeling like one has any control over the home environment, not feeling listened to, and of course, feeling endlessly judged by strangers about one's every move as a 'parenting person' in regards to the DSC. I think one thing that many people can't appreciate is how difficult is in to maintain one's confidence within a new stepfamily. Maybe, OP, if you look really hard at the other issues that are bothering you and begin to address those, you will feel better about this (non?)issue.
OP, for what it's worth, in my own experience (two DSCs already when DH and I had our DD), none of this was worth worrying about. My DH could not have been more thrilled with DD, could not have been more devoted to her, and he continues to be surprised by her every day. (Of course, today he was surprised by her saying "so then the fisherman threw it back because it was a fucking fish" - but I digress.) She is her own person, she has bits of DH and bits of me about her, and she is not just a tiny clone of his other children.
I was lucky, maybe, because I could talk to my DH about my own ideas and wishes as a new parent, without him becoming defensive or unpleasant about it. We talked about how decisions would be made, and what we'd do when we didn't agree about something in her upbringing - and also about what I needed from him during the pregnancy and first months. I was not a young mum, and I'd had a lot of experience looking after babies; I also had my own views about how I wanted to manage things like sleep routines, feeding, labour, and so on. And I pretty much told him that I appreciated his previous experience (I did and I do - it made him a very calm new dad, and that helped me and my DD to have a calm beginning), I did not want to hear endlessly about how he'd done things with his previous pregnancies and babies, that I was going to make mistakes and discover things for myself, and I wanted that to be ok. He understood that, and I had a really good experience of new motherhood.
The details about where the baby will sleep and so on - these are a different matter. Of course they need to be worked out, and there are plenty of threads on here already about the bedroom issues that come with having more children (whether all there at once or not) - do read through them.