So I do take part in some things... but as her Father he has all final say. That comes from his growing up too, as it was always the man that dished out discipline and set it and so forth.
Yikes! This would be a major red flag for me. It sounds like something that definitely needs to be worked out *before* the two of you get married, unless you want to feel like a second-class citizen in your own home. If you can't parent together with the stepdaughter, how will you parent together with a new baby? Will you have two sets of rules for the two kids? Or will you have to do everything for the new baby the same as the older child (i.e., as your DH and his ex have deemed appropriate) to have consistency? I'd be very wary of this arrangement. Unless you want him dictating all parenting decisions to you, then you need to sort that out ahead of time. Think about what it is you want and envision for your future and then have a long talk (or several) with him to make sure you're on the same page. If you have a child and then find out it doesn't work, you could be in a much worse situation. Best of luck.