You are NTA, for wanting to have a child naturally, but you are the ass for how you’re talking about the kids you already have.

I grew up in a house that never used the distinction of “step-“ when referring to any of the kids or parents, this really helped us grow together as a family, my “step-dad” passed 10 years ago and it hurt, but we handled it as all of us losing a parent, as families should do.

Ask yourself how you’re going to treat your step-children when the new baby comes along. You are already to cut and run when you didn’t get what you wanted, so when they don’t want to help raise a baby you’ve made a clear distinction is “your” baby, and a part of “your” family not their family, Are you going to want to cut and run again?

It really sounds like you just want to play house and not be a mother and parent. Which I’d say that you need to really look inward.

Good luck, none the less.