Hi J.!
I see you have several responses about your situation. I hope that I am able to add something that could help heal your heart a little.
I am a stepmom. I have been for 14 years now. I fell in love with my stepkids during the first year of our marriage, and I'm really glad those relationships came before OUR first child together. It was because I nutured my relationships with my stepkids first, that we had hardly any troubles when our first son came along.
I do say "hardly any" troubles, and let me tell you why. My stepkids were 8 (girl) & 10 (boy) when I met them. The boy was totally fine with anything, but the girl was very used to being Daddy's little girl & Daddy's Princess, and in her young mind, a little baby threatened that relationship. She didn't understand that Daddy's heart had room enough for everyone.....Until I drew a picture of a heart (Daddy's heart), and divided into sections for the Refridgerator. The top part of the Heart was the largest sections and that was for my stepkids, so they "thought" that their Daddy had more love for them. This developed a natural security for everyone, and my step-daughter began to evolve into a beautiful big sister instead of a resentful one (because she was both resentful & wonderful).
There will definitely be challenging times....times change, rules change, children are different and respond to different forms of discipline. With all this combined with stepkids, there will definitely be challenges ahead of you.
I recommend that you sit down with your husband, and tell him that you have "bonded" with his children beautifully. Ask him to please not be the one to deny you the opportunity to "complete" you by not giving you your own childbirth experience.
If he says yes, then you have to promise yourself to work harder when your baby comes. You can't make them "wait" for their baby brother/sister very often. You can't miss a soccer game because of your baby, you can't change your specials "dinners" or whatever.....Make sure that your stepdaughters know & "feel" #1 in your life. Like nothing has changed for them. They will be better big sisters....I promise....
Good Luck, J., with your new life. It sounds like you have a wonderful situation. I hope that you and your husband can come to an agreement on this without too many problems.
I hope my experience has helped guide you to making a good choice for yourself....one way or the other.
Best Wishes,
:o) N.