Welcome, Chachazest! While I haven't experienced this exactly, I have a similar story to share. When I was 9, my mom & stepdad announced they were having a baby. Part of me wanted to be excited (I had been an only child for 9 years!), but part of me was offended (I had been an only child for 9 years!). I felt like they were replacing me with a baby that was all THEIRS, instead of part of someone else. I don't think they really took my feelings into consideration, just expected me to accept it. (You & your DH are way ahead of the game in that respect!) I was a shy little girl, and didn't say much about it, so they probably figured it was fine...but really, it pushed me away. Especially when they expected me - at 10 - to take care of the baby while they did yard work or something. I felt like a built-in babysitter instead of part of the family, was very hurt by their lack of recognition of my feelings, and it sent me on a bad emotional trip that I didn't pull out of until my 20's.
Now I'm all grown up, with a blended family of my own...DS 1 & 2 are from a different father, who thankfully is no longer in the picture. When DH & I had our DD, we made sure to include the boys and show them just as much love. We talked a lot about the baby, and the fact that we're all a family now...we even included the kids in our wedding and had the pastor say a little piece about us being joined as a family. They have accepted their stepdad is here for keeps and loves them...they even call him Daddy now, and all the kids are thrilled at our upcoming new addition!
I guess my advice for you would be, since you know your stepdaughter has difficulty expressing her feelings, anticipate them! You might pleasantly surprise her, and make it easier for her to open up if she feels you "get" her without her having to say anything. Maybe you could start by telling her you know this whole blended family situation isn't easy, and it's totally normal for anyone to feel that way... and you love her unconditionally. Tell her she's actually lucky in a way, because now she has 2 moms to love her - one who gave birth to her, and one who chose to be her mom. Even if she doesn't say much, talking to her like that will do wonders! Show her as much genuine affection as possible; I'm talking crazy big group hugs with Dad, tickle wars, pillow fights, anything!! When kids have a hard time opening up, they need to feel showered in love and acceptance, and even if they seem to resist it, that's what they need the most. It makes me think of a quote from Dr. Aletha Solter: "Children are the most in need of loving attention when they act the least deserving of it."
When you tell her about the baby, make sure she knows this baby is only multiplying the love in your growing family, and is a part of her, too, since they share a daddy. And she will have the incredible role of big sister, for this little one to look up to adoringly! Tell her that her place in your hearts is secure as she will always be your #1 little lady no matter what. Take things at her pace, and don't expect an immediate positive response. She'll come around as long as she feels loved, and when she's ready, involve her in decisions about the baby, like what colors to use for the nursery, or ask for her favorite baby names. Show her the pictures of fetal development so she understands what's going on with her little sibling. Kids are naturally curious about this type of stuff, and it really gets them excited about the baby.
Hope this helps - good luck to you!!